It's difficult to say when adulthood truly starts. I joined the Wardens at age eighteen, though by then I'd already seen quite a few battles already. Your first time in combat certainly makes a man out of you... though that age can obviously vary.

Marriage can happen in the teenage years, though it seems more common into the twenties. It all depends, sometimes nobles decide to marry off their children young for some political reason. It doesn't always happen, but it can.