Learn English – Word opposite to “warm-up” in sports

I suggest hard-won, though this is not noticeably any better than hard-gained or hard-earned.

Note however that hardly should not be employed. Hardly usually means “just barely” or “almost not” rather than “with difficulty”. It is therefore conventional with participles to employ hard-, hyphenated, to express the meaning “with difficulty”.

The "opposite" of a 'minor' is an 'adult'. Or, you can say "she has reached the age of majority." For the latter see the Oxford dictionary, definition 2.0.