What Goes in a Message Your Child Opens at 21
The hard part is not the sentiment. The hard part is that you are writing to somebody who does not exist yet.
The child you know is eight. The person who opens this is an adult you may never meet, with a job, opinions, possibly their own children, and a memory of you that has been doing its own work for years without your involvement. Almost every mistake people make with these messages comes from writing to the first person instead of the second.
Write to the adult, not the child
The most common version of this is a message addressed to a little kid. Nicknames, baby talk, references to a toy, a tone pitched at bedtime.
Read at 21 that lands oddly. It can feel like being handed a photograph of yourself you do not remember being taken. The writer is clearly addressing somebody else, and the person reading has to translate.
Write as though you are talking to a capable adult you like. You can mention the eight year old. Just do not address them.
Do not put conditions in it
This is the mistake with the longest tail, and people make it with the best intentions.
"I hope you've finished your degree by now." "I know you'll look after your brother." "Don't let anyone talk you out of the music."
Each of those sounds like encouragement when you write it and arrives as a verdict. The reader has one life, it did not go the way anybody planned, and a message from someone who cannot be argued with is not a conversation. It is a standing instruction with no appeal.
The useful rule: nothing in the message should be something they could fail.
What actually works, in rough order
Specific memories with details only you have. Not "you were such a happy baby." What you noticed. The thing they did at four that you still think about. The sentence they said in the car. People keep these, because they are information rather than sentiment, and because nobody else on earth can supply them.
Permission. The single most valuable thing in these messages, and the most frequently left out. Tell them what they are allowed to do. Sell the house. Stop visiting. Use the money on something frivolous. Not think about you on the anniversary. Grown children carry imagined obligations to dead parents for decades, and one sentence can lift something they were never actually asked to carry.
Facts about where they came from. The people they never met, and what those people were like in ways no document records. Why the family left wherever it left. What your mother was actually like as opposed to the official version. This is the material that becomes irreplaceable fastest, because it dies with the last person who holds it.
The things you got wrong. Said plainly, without asking for anything. An adult reading an honest account of a parent's mistakes tends to find it a relief rather than a burden. It means the person writing was real.
One piece of advice, maximum. Resist the collected wisdom. A list of twelve life lessons reads as a speech and is remembered as a genre rather than as you. One thing you actually believe, in your own words, is worth more than all of it.
Keep it short, and write several
Long messages do not get finished. That sounds brutal and it is simply true, and it is especially true of a message read in a state of some emotion.
Three hundred words that are specific will be reread for years. Three thousand words of accumulated thought will be read once, carefully, and then kept somewhere safe and not opened again, because opening it is a whole event.
If you have a lot to say, the answer is several short messages at different ages rather than one long one. They also read very differently when they arrive apart. A short note at 21 and another at 30 feels like someone who kept in touch. One long document feels like an archive.
Say how old you were
Date it, and say what was going on when you wrote it. It is a small thing and it changes how the message reads.
A reader who knows you were 38 and worried about work and writing this at the kitchen table after everyone went to bed is reading a person. A reader with an undated message is reading a monument. The first is much easier to be close to.
It also ages honestly. If you wrote it fifteen years before they read it, the message gets to be fifteen years old rather than pretending to be timeless.
Write it badly, now
The thing that stops people is wanting it to be good. So it stays notional for years, and the version that exists is no version.
Write the bad one this week. It will be shorter and more direct than the one you were planning, which is an improvement on every axis. You can replace it whenever you like, and almost nobody does, because the plain version turns out to be the one they meant.