I'm 20.
To get back on topic, if someone is happy in themselves then that is a good thing, and not everyone IS happy. However, if they are secretly sad, you could address that but you have to do it eg telling them they're hermits, need a job etc will only make them withdraw more, into their own world, where they may be happy or sad or any combination of emotions.
So I would say encourage them, try to keep positive for yourself and for them. Sooner or later, they will discover what they want to do with their lives, and will be even happier they found it out themselves.
Good luck.