Money, capacity and the eighteenth birthday: what changes when your child becomes an adult in law

For a SEND family, the eighteenth birthday is not really a milestone. It is a legal threshold with a party attached to it.

I appreciate that is a bleak way to open. I would rather be bleak in an article you can read in July than watch a family discover the point in October, standing in a bank, being told with genuine sympathy that they are no longer authorised to discuss their own child’s account.

Most parents mark eighteen as a milestone of growing up, and rightly. For families whose child has significant additional needs, it is also a date that arrives whether or not the young person is able to step over it. On that birthday the law treats your child as an adult with the right to make their own decisions. That is true even where, in practice, your child cannot manage their own financial or legal affairs.

The gap between the legal status and the practical reality is where families get caught. And the time to understand that gap is well before the date, not in the confusion that follows it.

The assumption that quietly ends

All the way through childhood you make decisions for your child as a matter of course. You manage their money. You sign their forms. You speak to their bank, their school and their services, and nobody asks you to justify why. The law assumes parental authority over a minor and everything is built on that assumption, including a great deal of ordinary daily life that you have never had cause to think about.

Think about how much routine business rests on it. The bank. The benefits correspondence. The conversations with health and care services where somebody currently takes your word for it that you are the person to speak to. None of that was ever formally granted to you. It came with being the parent of a child, and it goes when the child stops legally being one.

At eighteen that assumption ends.

Your child is legally an adult, and your automatic authority to act on their behalf falls away regardless of their actual capacity to take it up. If your young person cannot manage their own affairs, you do not simply continue as before. You have to be granted the authority through a legal process.

That is genuine news to a great many parents, and it tends to arrive at an inconvenient moment. Nobody sends a letter in advance explaining it. The change is silent, automatic, and dated.

Capacity is decision specific, which is more useful than it sounds

The framework here rests on the concept of mental capacity, and there is a subtlety in it that matters.

Capacity is not a single switch, on or off, applied to a whole person for life. It is assessed in relation to specific decisions at specific times. A young person may have capacity to make some choices and not others, and may have capacity for a decision on one day in one setting and not in another.

That is not legal hair splitting. It shapes what authority is actually needed, and when.

The law starts from a presumption of capacity and only departs from it where a proper assessment shows that the person cannot make a particular decision. So the question is never “does my child have capacity”, in the abstract. The question is always which decisions, and on what evidence.

Understanding that distinction helps you engage with the system accurately instead of arguing from an all or nothing position that the framework does not actually use. It also protects your young person from having more autonomy removed than the situation requires, which is a thing worth caring about in its own right.

Deputyship and the Court of Protection

Where a young adult genuinely cannot make financial decisions for themselves, the usual route to managing their affairs is through the Court of Protection, which can appoint a deputy, very often a parent, to act on their behalf.

I am not going to pretend this is quick or trivial. It takes time. It has costs. It comes with ongoing responsibilities and with oversight, because you are being trusted with somebody else’s money and the system quite reasonably wants to know how you are handling it.

And it is precisely because it is not instant that knowing about it in advance is so valuable.

A family that understands deputyship before the eighteenth birthday can start the process in good order, at a sensible pace, with the paperwork assembled and somebody sensible advising them. A family that discovers it the week they cannot access their adult child’s account is doing the same process in crisis, on a deadline, while also doing everything else they were already doing.

It is also worth knowing that deputyship is not the only mechanism in this area, and that what suits one family will not suit another. A young person who can make some decisions may need something narrower than a full financial deputyship. That is a conversation to have with a specialist solicitor rather than with an article, and it is a much better conversation to have early, when the options are still options.

Same process. Completely different experience of it.

Start reading about it a year out. That is not over cautious. It is roughly what the timescales deserve.

How it connects to everything else

This transition does not happen in isolation, which is the part families most often miss.

It runs alongside the benefits shift, including the move from Disability Living Allowance to Personal Independence Payment at sixteen and the way adult means testing assesses capital and income differently from the arrangements you have got used to. It connects directly to the trust planning that protects means tested support, because adult means testing is exactly what that planning exists to deal with. And it sits inside the wider preparing for adulthood transition which, done properly, starts years before any of this.

So the eighteenth birthday is one node in a web of changes. Every strand of that web rewards early preparation and punishes the last minute, however unfair that is on families who have no spare capacity for early preparation and nothing left over for anything.

The families who handle it best are not the most organised by temperament. They are usually just the ones somebody warned in time.

Plan for the threshold, not the surprise

The practical message is simple, even though the law behind it is not.

Do not let the eighteenth birthday be a surprise. Learn in advance what changes about your authority. Understand what mental capacity means in law, and specifically that it is decision by decision. Find out what deputyship through the Court of Protection involves, and whether your young person is likely to need it. Take advice from somebody who does this work, early, while the date is still a date rather than a problem.

Write the date down somewhere with a reminder set a year before it. That sounds trivial. It is the single most effective thing in this article.

None of this diminishes the milestone. Your child becoming an adult is still worth marking, and I would encourage you to mark it properly, cake and all. But for SEND families it carries a layer that other families never have to think about, and meeting that layer with preparation rather than panic is one more instance of what SEND parents do every day, which is look ahead on behalf of somebody who needs them to.

It is not the eighteenth birthday anybody imagines when their child is small. It is the one you get. You plan for it, you get through it, and then you have the party.

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Trusts are not regulated by the Financial Conduct Authority.

This article is general information drawn from professional and personal experience and is not legal advice. Capacity, deputyship and Court of Protection matters are complicated, so take specialist legal advice for your own circumstances.

Legacy Capital Management is an Appointed Representative of and represents only St. James’s Place Wealth Management plc (which is authorised and regulated by the Financial Conduct Authority) for the purpose of advising solely on the Group’s wealth management products and services, more details of which are set out on the Group’s website at www.sjp.co.uk/products.

The ‘St. James’s Place Partnership’ and the titles ‘Partner’ and ‘Partner Practice’ are marketing terms used to describe St. James’s Place representatives.

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SJP Approved 30/07/2026