Apply minimum ages to computed carer and DLA mobility awards

changelog.d/1793.fixed.md

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+Applied the minimum ages to computed Carer's Allowance, Carer Support Payment and DLA mobility awards, preserving DLA care and explicit amount overrides.

changelog.d/2474-combined-csp-gates.fixed.md

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+Retain both the minimum-age and earnings conditions when combining Carer Support Payment fixes.

docs/book/assumptions/benefit-minimum-ages.md

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+# Benefit minimum ages
+
+Computed Carer's Allowance, Carer Support Payment (CSP) and Disability Living
+Allowance (DLA) mobility awards apply these minimum ages to each person's existing
+annual `age` input:
+
+| Computed benefit | Minimum age |
+| --- | ---: |
+| Carer's Allowance | 16 |
+| CSP | 16 |
+| Higher DLA mobility component | 3 |
+| Lower DLA mobility component | 5 |
+
+The Carer's Allowance condition appears in section 70(3) of the
+[Great Britain Act](https://www.legislation.gov.uk/ukpga/1992/4/section/70) and its
+[Northern Ireland counterpart](https://www.legislation.gov.uk/ukpga/1992/7/section/70).
+CSP's condition is in
+[regulation 4(1)](https://www.legislation.gov.uk/ssi/2023/302/regulation/4).
+The DLA mobility ages and corresponding higher/lower routes appear in sections
+73(1), (1A) and (11) of the
+[Great Britain Act](https://www.legislation.gov.uk/ukpga/1992/4/section/73) and
+[Northern Ireland Act](https://www.legislation.gov.uk/ukpga/1992/7/section/73).
+Other qualifying conditions remain necessary. These gates do not implement a full
+eligibility assessment or change education, caring, disability, duration, upper-age
+or programme-replacement rules.
+
+## Reported inputs and amount overrides
+
+A positive `carers_allowance_reported` value or sufficient caring hours cannot
+bypass the age condition in the computed CA/CSP pre-overlap amount. A DLA mobility
+category similarly cannot bypass its category's age condition. The raw reported
+amounts and categories remain stored inputs.
+
+Explicitly supplying a pre-overlap or final amount keeps the model's generic input
+override behavior. For example, an explicit `carers_allowance` or `dla_m` amount
+can override its formula. Such an override is not evidence of statutory
+entitlement. Report-only positive carer cases also preserve an existing proxy;
+reported receipt does not legally waive the qualifying-care requirements.
+
+DLA care has no categorical minimum-age floor. Its separate needs, duration and
+child-comparison conditions still apply, as set out in
+[section 72](https://www.legislation.gov.uk/ukpga/1992/4/section/72) and its
+[Northern Ireland counterpart](https://www.legislation.gov.uk/ukpga/1992/7/section/72).
+A care category for an infant assumes those conditions; it is not automatic
+entitlement. DLA totals continue to add the care and mobility components.
+
+## Dates and the annual model
+
+The CA age parameter begins on 1 January 2015 as the model's existing coverage
+anchor. This is not a claim that the legal age condition commenced in 2015.
+CSP's raw age parameter starts on 19 November 2023, the commencement of regulation
+4. The DLA mobility parameters start on 9 April 2001, as recorded in the Acts'
+amendment annotations. These dates do not fill missing historical benefit rates
+or expand the existing Scottish CA-to-CSP model switch from 2025.
+
+`age` is a Person/YEAR floating-point stock, with an existing default of 40 when
+omitted. The gates compare it directly, without flooring or rounding fractional
+ages. Awards retain the existing annual rate-times-52 convention. They do not
+calculate exact birthdays, part-year awards or award weeks. In particular, this
+does not newly implement CSP regulation 4(2)'s leap-day birthday rule.
+
+At the unchanged 2026 rates, an otherwise qualifying carer aged 16 receives
+£4,495.40 before overlap; an under-16 computed award is zero. Higher and lower DLA
+mobility amounts are respectively £4,160 and £1,575.60 annually once the relevant
+age floor is met. These conditional examples do not estimate a population effect
+or establish independently encoded parity.
+
+## Combined CSP conditions
+
+This change is stacked on the repaired [PR #2473](https://github.com/PolicyEngine/policyengine-uk/pull/2473).
+Both the age condition and the earnings condition apply to computed CSP, including
+the reported-Carer’s-Allowance input proxy. Natural-pay cases combine age15,
+fractional ages below16, age16, the exact annual earnings limit and favorable
+fractional-penny treatment. The shared reader retains merged [PR #2444](https://github.com/PolicyEngine/policyengine-uk/pull/2444)
+NI-age deductions. Existing annual/award-week, PAYE and care-charge qualifications
+remain in the [CSP earnings assumptions](carer-support-payment-earnings.md).
+
+Only after #2473 actually merges should this branch be retargeted and revalidated
+on current main; a queue entry or passing checks do not establish that landing.
+The shared-parameter change in open [PR #2518](https://github.com/PolicyEngine/policyengine-uk/pull/2518)
+requires future CSP-reader/prose coordination; it is not copied or repaired here.

policyengine_uk/parameters/gov/dwp/carers_allowance/min_age.yaml

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+description: Minimum age for computed Carer's Allowance entitlement.
+# Existing model coverage anchor, not the statutory commencement date.
+# The age condition predates the model's supported 2015 fiscal year.
+values:
+  2015-01-01: 16
+metadata:
+  unit: year
+  period: year
+  label: Carer's Allowance minimum age
+  reference:
+    - title: Social Security Contributions and Benefits Act 1992, section 70(3)
+      href: https://www.legislation.gov.uk/ukpga/1992/4/section/70
+    - title: Social Security Contributions and Benefits (Northern Ireland) Act 1992, section 70(3)
+      href: https://www.legislation.gov.uk/ukpga/1992/7/section/70

policyengine_uk/parameters/gov/dwp/dla/mobility/higher_min_age.yaml

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+description: Minimum age for the higher DLA mobility component.
+values:
+  2001-04-09: 3
+metadata:
+  unit: year
+  period: year
+  label: Higher DLA mobility minimum age
+  reference:
+    - title: Social Security Contributions and Benefits Act 1992, section 73(1A)(a) and (11)
+      href: https://www.legislation.gov.uk/ukpga/1992/4/section/73
+    - title: Social Security Contributions and Benefits (Northern Ireland) Act 1992, section 73(1A)(a) and (11)
+      href: https://www.legislation.gov.uk/ukpga/1992/7/section/73

policyengine_uk/parameters/gov/dwp/dla/mobility/lower_min_age.yaml

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+description: Minimum age for the lower DLA mobility component.
+values:
+  2001-04-09: 5
+metadata:
+  unit: year
+  period: year
+  label: Lower DLA mobility minimum age
+  reference:
+    - title: Social Security Contributions and Benefits Act 1992, section 73(1A)(b) and (11)
+      href: https://www.legislation.gov.uk/ukpga/1992/4/section/73
+    - title: Social Security Contributions and Benefits (Northern Ireland) Act 1992, section 73(1A)(b) and (11)
+      href: https://www.legislation.gov.uk/ukpga/1992/7/section/73

policyengine_uk/parameters/gov/social_security_scotland/carer_support_payment/min_age.yaml

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+description: Minimum age for computed Carer Support Payment entitlement.
+values:
+  2023-11-19: 16
+metadata:
+  unit: year
+  period: year
+  label: Carer Support Payment minimum age
+  reference:
+    - title: The Carer's Assistance (Carer Support Payment) (Scotland) Regulations 2023, regulation 4(1)
+      href: https://www.legislation.gov.uk/ssi/2023/302/regulation/4