21 years is a long time to age whiskey—-I think that is a fantastic idea but you you will want to use a large barrel- I would follow what commercial distilleries are doing that age for 21 years. They are all using 53 gallon barrels for long term aging.
I would use an old barrel whatever size you decide on:
The spirits in a barrel expand in the hot summer and get pushed into the porous wood. In the winter, the spirits contract due to the cold, and wash back out of the wood. Through this process, some of the distillate evaporates- the angel’s share. you will lose about 2% of the barrel each year. However, it varies from state to state and environment to environment. In the south it can be as high as 6%.
longer age times don’t always produce better spirits. Most of the aging is taking place in the first two years. Most distillers would never age an American whiskey for more than 8 years. It’s really between 2-8 that you get a perfectly aged American whiskey.
Scotch can last much longer in oak because the barrels aren’t new, the spirit needs more mellowing, and it can stand up to a longer period of time in the barrel.
I like the idea…I just don’t know how practical it will be— you could use a smaller barrell (5 gallon or less) and age it for a shorter time and then bottle it…storing it in a cool dark room until he is 21.