If somebody you knew was thinking about joining the army at age 25, what would you tell them?
If that's what you want to do, then do it. If that's not really what you want to do, maybe think about it some more. Research it, talk to friends/family about it, find someone who's been in the military and talk with them about it (if they want to), and don't put too much stake in the opinion of some dude on the Internet.