I'll actually answer seriously, because I can relate to the subject very closely right now.
I don't quite fit the age parameters you've determined, but I started smoking when I was 14 and am a month or so away from being 21. I've been smoking consistantly the entire time, sans a three/four month period two summers ago because of a job.
Now people do have their different versions of 'regular' smoking. My first two years I smoked about once a day. For the next two years I probably smoked 2-3 times most days. In the last two years I've built up to smoking 5-6 times a day or more.
Here's my take on it. It's a great recreational drug. When you first start, you get an extremely euphoric and unexplainable feeling. Think of everything being just like it is sober, except for way cooler. When you hear about people bugging out or getting paranoid, it's probably because they're immature or just naturally shook. Plus, (and this depends of the quality of the bud and tolerance to weed,) it's not a feeling that lasts as long as most other drugs... INCLUDING alcohol. It should generally be only a 1-3 hour affair, but again, this can change based on many factors and the high isn't necessarily confined within those boundries.
Plus, it's really easy to disguise if you're young. Rohto V eyedrops, scentless Febreeze and you're pretty much set. That, and if you're really crafty, you can smoke whenever you want if you live with your parents. (Think of a place with really hot water and steam...)
If you've ever had alcohol, and are scared to try pot, that's just not right dude. There's a large majority of people who recognize the FACT that marijuana is a shit load safer than alcohol. If you're worried about cancer, there still haven't been any conclusive studies linking the contents of marijuana and lung cancer. The carcinogens the cause cancer in cigarettes just aren't found in weed, so there's that.
But I digress... you ask about how it's affected me.
I'm calmer, I see things from different perspectives, I like to think it helps keep me level-headed.
Set-backs: If utilized heavily (like me,) it CAN cause dangerous effects, like MUNCHIES!!!
Haha, but seriously, when I smoke heavy I can get winded a bit easier, and feel an EXTREMELY small drop-off in short-term memory. You'd have to smoke really consistantly and heavily for these to ever happen to you, though, lol.