You're certainly the more serious person in this friendship, and may feel a sense of responsibility or duty towards your friend. But when they try to assert the fact that they're an individual, you could find it hard to grant them the space and freedom they need. When you see them behaving in ways you don't approve of, you'll let them know about it.
Hey, you're just trying to help them mature and become a better, more stable person, right? But that's not how they'll take it. To them, any criticism, subtle or overt, will feel like a limitation to them, and one they could resent. Even well-intended advice won't always be welcome. They aren't looking for another parental figure, after all; they're looking for a friend and an equal. So don't take that dynamic too far. Be their equal, even if you're older or have had more life experience than they've had. They'll mature in their own time, and since this friendship has a good chance of lasting long-term, so you've got little to worry about in that respect.