These parts of your post stuck out to me:
Yeah, I had a girlfriend when I was 15. I'm out but I'm subtle. No-one guesses me as gay. I might not be the greatest looking or the smartest guy in the world, but I don't think I'm unattractive.
I was sort of dating a guy about a year ago, but he had a boyfriend the whole time and didn't tell me. I stop seeing him when I found out.
... but is it a common thing for gay guys to be late bloomers?
Firstly a lot of gay guys suffer (for lack of a better word) from delayed adolescence or in other words we're a couple years behind in experience with the dating game.
That's not to say straight people don't experience delayed adolescence- they do. But being closet for so many years can make you realize you missed out on a lot of stuff and have to play catch up. There's no rule on when people should start dating or be in some sort of relationship but we all have a general idea of how things progress with dating and when people "should" meet their first real BF. I'll stop there because that's a whole 'nother thread.
Secondly, nothing wrong with being subtle or "straight acting" but you might need to put yourself out there a bit more. Maybe even drop a hint or two in conversation. Nobody really cares if you're gay these days anyway unless you live in a small closed minded town. The one's that do are most likely closet cases IMO.
You're good looking and have a nice personality, seem to be well rounded in interests, etc... in other words I'm sure a lot of guys would be lining up to date you.
I believe in being proactive about dating and meeting your future BF half way. The belief that he has to do all the work and "come" to you seems more based in a chance meeting fantasy than reality. Dating is also a numbers game, the more guys you meet the more likely it is you'll find someone you just click with.
How are you meeting guys? What places do you go on a regular basis on the weekends? Are you home alone on the weekends or making an effort to get to there? Are you a member of a club or organization? I know a lot of guys aren't into the bar and club scene but you should try to go to one that fits you where you can chat with guys in a relaxed fashion without thumping music in the background and not being able to hear a word their saying. There's all types of bars.
Finally, I'm going to suggest checking out meetup.com. That's a great alternative to online dating because you can find other like minded guys who are interested in the same things you are in a lower pressure environment than a bar or club. You'd just be looking to meet friends at first, or even just go to get out and enjoy yourself. I'm sure you can find another like minded guy at one of those meetups.
Finally don't put too much pressure on yourself and let yourself fall into getting desperate, just get put yourself out there more. Sooner or later someone will recognize you are a catch. Don't worry he's out there looking for you.
Dating can be hard, for guys both gay and straight. I know how hard it is, I moved to NYC and I'm still single (like tons of other people in this city) But nothing worth doing is that easy. Hope that helps.