What Happens to a Junior ISA or Junior SIPP When Your Child Turns 18?

Both a Junior ISA and a Junior SIPP undergo a specific transition when a child turns 18 - but the two accounts behave very differently, and understanding this in advance helps set realistic expectations for both parents and the young person themselves.

Junior ISA: full access, immediately

At 18, a Junior ISA automatically converts into an adult ISA, and the money becomes entirely and immediately accessible to the now-adult child - the parent has no further legal control at this point, regardless of how the money was originally intended to be used. This is a genuine, complete handover: the young person can withdraw, spend, or continue investing the full balance however they choose.

Junior SIPP: control transfers, but access doesn’t

A Junior SIPP behaves differently - at 18, the young person takes over management of the account (choosing investments, for example), but the money remains completely inaccessible until they reach the normal minimum pension age, decades later - which is exactly why starting a Junior SIPP as early as birth makes such a disproportionate difference to the eventual sum. This is a meaningful distinction: control of the account transfers at 18, but the money itself doesn’t.

Why the JISA handover deserves preparation, not just a surprise at 18

Since a JISA gives complete, unrestricted access the moment a child turns 18, many parents find it valuable to have a conversation about the account’s existence and purpose well before that point (see our dedicated article on saving for university, which covers a similar timing consideration) - rather than the balance being either an unexpected surprise or, conversely, spent immediately and impulsively without any prior discussion about intended use.

From age 16: the child can start managing the JISA themselves

Two years before the full handover, at 16, a child can take over day-to-day management of a Junior ISA - checking balances, and in some cases directing how Stocks & Shares JISA contributions are invested - even though they still can’t withdraw any money until 18. This can be a useful, lower-stakes transitional period for building familiarity with the account before gaining full access.

What if you’d prefer more control over how the money is used at 18?

There’s no legal mechanism to restrict a JISA’s access beyond 18 - once it converts, it’s the young adult’s money entirely. Parents wanting more structured control over timing or use of a larger sum sometimes instead use a bare trust or discretionary trust (see our Inheritance series article on trusts) as an alternative or supplementary structure, though this introduces genuine additional complexity and cost that should be weighed against a straightforward JISA for most families.

What happens practically at the point of conversion

Most JISA providers handle the conversion to an adult ISA automatically around the child’s 18th birthday, sometimes requiring the young person to complete some updated identity verification or account paperwork in their own name for the first time - worth checking with your specific provider what steps (if any) are needed around the birthday itself.

The bottom line

A Junior ISA hands over both control and full access at 18; a Junior SIPP hands over control at 18 but keeps the money locked away for decades more. Understanding this distinction well in advance - and having an age-appropriate conversation about the JISA specifically, given its complete accessibility at 18 - helps both parents and young people approach the transition with realistic expectations rather than surprise.