Dear [child's name],
When you were [age] years old, I opened an account for you. It is called a Junior ISA. You are reading this because you have just turned 18, which means the account now belongs to you.
Here is what I did and why.
Every month, I put [£X] into the account. I did this because of a piece of maths that most people know about but few act on.
If you invest money in a stocks and shares fund and leave it alone, it grows. Not every year, and not in a straight line, but over long stretches of time the stock market has averaged returns that mean money roughly doubles every ten years.
That is the whole secret. Time and patience.
The account has had [N] years to grow. What is in it now is roughly [£X]. I cannot know the exact figure because markets move, but that is the rough shape of it.
Now the choice is yours.
You can take the money. It is yours, and I will not tell you what to do with it. But I would ask you to look at one number before you decide.
If you leave it invested until you are 67 -- just leave it alone, add to it if you can, do not touch it -- it could be worth [£Y]. That is not a trick or a forecast. It is the same maths that got it here.
The account is a Stocks and Shares ISA. The money inside it has never been taxed, and it never will be. You can move it, add to it, or leave it exactly where it is. You cannot put it back into an ISA once you have taken it out, so if you do decide to use some of it, use it for something worth it.
I started this because I wanted to give you a head start. What you do with it is up to you.
With love,
[Your name]
Note: the figures in brackets are yours to fill in. The letter works without exact numbers too. The point is the explanation, not the precision.