marked

adjective6

1. •
   Having a visible or identifying mark.
2. •
   very clear and easy to notice

   • The eighth century BC saw a marked increase in the general wealth of Cyprus.
3. •
   Having a secret mark on the back for cheating.
4. •
   In police livery, as opposed to unmarked. (of a police vehicle)
5. •
   Singled out; suspicious; treated with hostility; the object of vengeance.

   • A marked man.
6. •
   Distinguished by a positive feature.

   • "Young" is the marked element of the old/young pair, since the usual way of asking someone's age is "How old are you?".