Harry Potter joined the Auror department at the age of 17.
Everyone who participated in the Battle of Hogwarts was made an honorary Auror, and were invited to become actual Aurors if they wanted. Harry and Ron at least took up the offer.
As an added note, Hermione also joined the ministry, and became an influential civil rights activist, campaigning to reform the abusive treatment of other magical creatures by wizards. Specifically house elves, and to a lesser extent, goblins.