Since the OP is over a month old I dont know if the original poster is still monitoring but....

Given the issues with corks some have been having I cant really recommend this... But if you do store it upright (alcohol will attack the cork otherwise) in a dark place (light is bad for the scotch) and hope the cork doesnt dry out and fall apart on opening.

I think I would be more inclined to...
1. Buy a 21yo bottle of Scotch for their 21st birthday party.

2. More special... Buy a 21yo from a "Single Cask" Offering that was casked in their birth year. This may not be possible if they are not born in the later part of the year. It will need time to be bottled and make it through distribution channels.

Notes:
"Single Malt" refers to product of only one distiller. Most are "Vatted". ie several cask are mixed together (to try and keep it consistent from year to year) and allowed to normalize fro 6 months or so in a cask. The age date is for the youngest scotch included.

"Single Cask" offering means it all came from a single cask and so will all be the same birth year and their are only the number of bottles available that were in that cask. How many will depend on the time aging and what alcohol per volume its released at.

Single Cask offerings are often through independent bottlers or exclusive to certain distributors.