I'm not a lawyer but I do know there are different ways to do it. The first would be to seal it in a package of some sort leave instructions in your Will as to want you want done. You could set it up with whoever you trusted to hold it. A friend, the lawyer (who I'm sure would agree on a fee).
Another would be to set up a trust with a bank (if there is any money) with an executor who manages the trust. I'm sure as part of it you could have the package put in a safety deposit box and the trust would be charged for the rental of the box. Again you can set an age limit with this.
This is just the basics for exact details you'll need a lawyer or trying doing a search on the web. I'm sure the information is on it some where.
You can pretty much specify whatever you want. My father died first. The will was set up that mom got everything and then after her death the kids split evenly. However there were different personal items, jewelry, china, silver etc that she had just written down on a piece of paper as to who was to get what. As long as none of us contested it was considered a part of the will. It was in her own handwriting so that proves they were her wishes.
