A survey of the categories of claimant entitled to universal credit and exceptions to the rules.
Regulations set out the conditions for entitlement. The claimant must: 
be aged 18 or over (with limited exceptions) and under the qualifying age for pension credit
be habitually resident in the Common Travel Area (UK, Channel Islands, Republic of Ireland or Isle of Man) with a 'right to reside' and not subject to immigration control. In the regulations the category of ineligible claimant is described as 'persons treated as not being in Great Britain'
not be in education, with limited exceptions
accept the 'claimant commitment' (for further information on this, see the page Claimant commitment).
Persons from abroad
Regulations set out the eligibility requirements for persons, including British citizens, who have come to the UK from abroad.  The categories of persons from abroad who are ineligible for universal credit are broadly the same as those who are ineligible for housing benefit.
Non EEA/EU Nationals
A person can only apply for Universal Credit if they are not subject to immigration control. There are some limited categories of persons from abroad who may be entitled to universal credit. However a person with ‘no recourse to public funds’ or who is subject to immigration control may be putting their immigration status at risk if they apply so it is essential to seek further advice from an immigration advice specialist if this might be the case.
EEA nationals who are a jobseeker in the UK (or their family member) are not entitled to Universal Credit (UC) unless they also have a different right to reside as any of the following:
a former worker
a person who has a permanent right to reside
a family member of one of the above
the main carer of a child who is at school
The DWP has issued Memo ADM 14/18 giving guidance on Commonwealth citizens who are claiming universal credit, and are long term-residents in the UK, but are unable to provide documentary evidence of their immigration status. This advises that 'where the evidence of nationality and status provided to and held by DWP and the Home Office indicates that the balance of probabilities is that the claimant is likely to have legal and habitual residence, the decision maker may determine that the claimant is eligible to claim...'. This is not restricted to the ‘Windrush generation’ (migrants from Commonwealth Caribbean countries who settled in the UK before 1973), see para 4 of the Memo for details.
Afghan citizens will be eligible to apply for means-tested benefits where they have settled in the UK in connection with the collapse of the Afghan government on 15 August 2021. Those clients can apply for universal credit, as long as they can show they are on a low enough income. The same applies to Afghan citizens in the UK under certain schemes:
the Afghan Relocations and Assistance Policy (ARAP)
the Afghan Citizens Resettlement Scheme (ACRS)
the previous scheme for locally employed staff in Afghanistan, sometimes referred to as the ex-gratia scheme
Any person living in the UK under these conditions can also apply for the following means-tested benefits:
Employment and Support Allowance
People resettling from Afghanistan under the schemes or in connection with the collapse of the government will not be subject to the habitual residence test. 
Eligibility for non-means-tested benefits will also apply, but only to those in the UK under one of the three schemes mentioned above. This will include benefits such as carer's allowance and personal independence payment. Local authorities have the duty to assist people with their benefit applications when they are subject to these conditions.
Claimants aged 16 or 17
A claimant aged 16 or 17 is entitled to claim universal credit if s/he: 
has a dependent child or in some cases is a member of a couple and the other member of the couple is responsible for a child
has been assessed as having limited capability for work, or is awaiting assessment
is caring for a severely disabled person
is pregnant and it is less than 11 weeks until the expected date of birth
was pregnant and it is less than 15 weeks since the date of birth
is without parental support. This includes where the claimant is estranged or at risk from her/his parents.
Most people in full-time education are not entitled to universal credit. The exceptions to this rule include a student who: 
is aged under 21 and in non-advanced education (or was under 21 when s/he started the course of study) and is without parental support
has a dependent child, or is a member of a couple in which the other member is also in education and has a dependent child
is a foster parent, or is a member of a couple in which the other member is also in education and is a foster parent
is entitled to attendance allowance, disability living allowance or personal independence payment and has been assessed as having limited capability for work
is over the qualifying age for pension credit and her/his spouse, civil partner or cohabiting partner is under that age.
18 to 21 year olds
From 31 December 2018, 18 to 21 years olds are entitled to universal credit housing costs element. 
From 1 April 2017 until 31 December 2018, for claims made in a full digital service area, claimants aged between 18 and 21 years were not eligible for the housing costs element of universal credit unless they were in an exempt group. 
Claimants of pension credit age
Single claimants over the qualifying age for pension credit are not eligible to claim universal credit.  At a date to be decided, pension credit will include:
a 'housing credit' element for pension credit claimants who need help to pay their rent , and
an extra amount for dependent children.
After this change, a claimant in receipt of pension credit will not be able to receive housing benefit or tax credits.
Mixed age couples
Where a claimant is a member of a couple in which one member has achieved the qualifying age for pension credit and the other has not ('mixed age couples'), the couple will be able to claim Universal Credit.  Until 14 May 2019, claims for those housing benefit and state pension credit age are still open to all people over state pension credit age.  This means that mixed age couples will effectively have the choice of claiming housing benefit and state pension credit or universal credit. Specialist benefits advice should be sought in this situation.
From 15 May 2019, the ability to claim pension credit and housing benefit will be withdrawn from mixed age couples and they will have to claim universal credit.  DWP have written to all known mixed age couples currently claiming pension credit and/or housing benefit to inform them that a change in circumstances which would lead to natural migration they will need to claim universal credit. 
Mixed age couples can continue to claim housing benefit, where either the older member is in receipt of severe disability premium (SDP), or the younger member is not eligible for UC due to one of the following:
s/he does not satisfy the habitual residence test
s/he is a person subject to immigration control
s/he is under 18 years old
s/he is in prison
s/he is absent from the UK for longer than the allowed temporary absence period.
Where the older member of a mixed age couple is in receipt of SDP, s/he would be treated as not having reached state pension age and would be able to claim working-age housing benefit. Where the younger member of a mixed-age couple is not eligible for UC for one of the reasons listed above, the older member is treated as a single claimant and able to claim pension age housing benefit. 
The Gov.uk state pension calculator can be used to calculate when a claimant will qualify for pension credit.
Last updated: 17 September 2021
regs 7 to 19 Universal Credit Regulations 2013 SI 2013/376
reg 9 Universal Credit Regulations 2013 SI 2013/376, as amended.
The Social Security (Habitual Residence and Past Presence) (Amendment) Regulations 2021
reg 8 Universal Credit Regulations 2013 SI 2013/376.
reg 14 Universal Credit Regulations 2013 SI 2013/376.
reg 4 Universal Credit and Jobseeker's Allowance (Miscellaneous Amendments) Regulations 2018 SI 2018/1129.
para 4A – para 4C, Sch 4, Universal Credit Regulations 2013 SI 2013/376 as inserted by Universal Credit (Housing Costs Element for claimants aged 18 to 21) (Amendment) Regulations 2017 SI 2017/252; ADM Memo 6/17.
s.4(1)(b) Welfare Reform Act 2012.
reg 3(2) Universal Credit Regulations 2013 SI 2013/376.
art 7(4) The Welfare Reform Act 2012 (Commencement No. 23 and Transitional and Transitory Provisions) Order 2015 SI 2015/634 and reg 6(3) Welfare Reform Act 2012 (Commencement No. 21 and Transitional and Transitory Provisions) Order 2015/33.
art 5 The Welfare Reform Act 2012 (Commencement No. 31 and Savings and Transitional Provisions and Commencement No. 21 and 23 and Transitional and Transitory Provisions (Amendment)) Order 2019 SI 2019/37.
para 14-17 LA Welfare Direct Bulletin 3/2019, DWP, 21 March 2019.
art. 2 The Welfare Reform Act 2012 (Commencement No. 31 and Savings and Transitional Provisions (Amendment)) Order 2019 SI 2019/935; A9/2019 Mixed age couples: further guidance, DWP, 21 May 2019.
- CPAG - advice line for frontline advice and support staff in Scotland
- Advice for decision making: staff guide
- Check your State Pension age
- Gov.uk - Housing Benefit for local authorities: adjudication circulars
- Revenue and Benefits - Universal Credit
- Universal Credit: information for new applicants