1. He already has a life plan. While an older man who's planned his life out will make room for you if he loves you, he's not really going to change his ambitions and goals for a relationship.
2. You can talk to him about your apprehensions. You're not expressing your worries just to get them off your chest. You're doing it because you know he's been through similar existential crises and difficulties. You believe he knows what to say to make you feel better.
3. He seems to know a lot more about life than you do. It comes with age, of course. And it is advantageous when you get the help and comfort you're looking for. Do take note, howevs: Unsolicited advice and being told what to do can be pretty annoying.
4. It's safe to assume he's dated and hooked up with a lot of girls. This comes with age too. He probably has a whole collection of high school and college sweethearts. You might not want to dig up the archives.
5. There's a big part of his life you missed, and he might not tell you much about it. Okay, you don't know if you should know every detail of his past, but you're curious. Was he a jerk when he was a teenager? Did he change to become a respectful man in college? How wild was he? The questions are endless. You can ask him, sure. Just nod and smile when you hear his answers. (Remember, that was the old him.)
6. He might not be as "adult" as you think he is. It's possible that he just wants to have fun with you and not have a serious relationship. It could also mean he still plays videogames and watches cartoons. That can be cute, though!
7. He's not the jealous type. He's got a lot on his plate to be having trust issues when your guy friends talk to you. He also understands that they're just your friends, who happen to be guys.
8. He's responsible (If you're lucky). Working for years and climbing several rungs have taught him a lot about responsibility and the importance of work. He's not the type to slack off, blame others for something he did, or take credit for something he didn't do.
9. He does household chores. Living on his own (provided he is) has left him no choice. Also, he can clean up after himself!
10. He's not always down for serious conversations. Just because he's all grown up doesn't mean he wants serious talk all the time. Most of the time he wants to talk about light things, because life is pretty heavy as it is.
11. He has a child-like side, and will show it only to you. All that corporate talk is just for show to his colleagues. If he loves you, he'll be comfortable showing this heartwarming silly side. Plus points if he baby talks. And if he loves you, it's for real.