but we just won't tell her about stocks
Unfortunately it doesn’t work like that, the financial institutions write direct to the child as they approach 18th and all parental access stops on their birthday. So you can’t just not tell them about it as some on here are saying, they will be told automatically by the bank or investment house.
In terms of investing till 21 or 25, the only way that I know of is to set up a full trust, with associated trust costs that could potentially eat away the money in the fund. There may be other ways of delaying money to 21 and 25 that I am not aware of but not many I suspect.
DC got an unexpected but quite large inheritance when they were 18 and almost 16. We had no control over when they got it as the distant relative that died didn’t stipulate an age and all other beneficiaries were older.
We did however have talks with them about how lucky there were, how it would be a substantial house deposit and how, if they wasted it, we wouldn’t replace it (even though we do have decent savings ourselves). It is being drip fed into their S&S ISAs and they have no interest in blowing it. If anything it has helped their understanding of money and investments.
DD was the only under-18 beneficiary when the estate was distributed and although the solicitors were officially the trustees, they spoke to us and said that the cost of them managing just her pot for 2 yrs would be a waste of money, so suggested passing it into the control of myself and DH. Even their costs in the 7mths from distribution to handing money to us were a decent amount, goodness knows what their trust costs would have been!
Whilst I can understand why people say no way, I would also say that with discussion and involvement, getting money at 18 doesn’t have to be a disaster and can help with financial understanding.