underage

Short definition from Wiktionary

Adjective

1. 1
   Below the legal age (usually age 18) for some activity, such as consuming alcohol or engaging in sexual intercourse.
2. 2
   Not yet a legal adult; still a minor.
3. 3
   Limited to players below a certain age; junior.

Noun

1. 1
   A deficit or shortfall in funds, inventory, or capacity.