OH! ages 5, 10, 16, and 21.
I'll let the real parents here suggest the best things for ages 5 and 10, but for age 16 I'd suggest some contemporary music CDs or DVDs that will probably have appeal - and maybe some classics, too - plus something about current cars, because that's an interest for sure at age 16! -- and for age 21, check with your local really good wine store - there are some good brandies and cognacs that will keep that long; they must must must be properly stored, though... assuming it is OK for alcohol presents to be given.
[I'm quite aware of the dangers of alcohol but I'm also aware that it is a lovely thing for those who are not endangered by it and know how to use it in proportion. I still miss the aperitifs and digestifs that preceded and concluded nice meals on the Continent...]
Also, I think family mementos are good for all ages, but the really significant ones are probably best held for the later years - for age 10, maybe you could give a letter about what it was like the day the child was born. For age 16, maybe a letter about how their parents met, how their grandparents met, how you met your husband, with pictures of each couple at their wedding. For age 21, maybe a genealogy... and a piece of heirloom jewelry?
hope this helps!
