No way am I telling you not to start! You seem like you'll have a ton of fun with it! =]
As for staring lessons, I have been at the same barn since I was 8. Now that we're looking into buying/leasing, I realized how completely overly expensive the leasing coasts and boarding costs are, and looked back on another barn I'd been looking at.. it's 250$ cheeper for boarding and a heck of a lot less for leasing. So I'd look at that if you think one day you might want to buy a horse.
I agree that you might want to look on a lesson & decide if you like the instructors style.
I don't see a problem with age at all. I know a some 50 year old who rides and she has great posture. She started when she was in her 30s, but hey, that's older than you are by a lot ;]
Keep in mind that if you're doing lessons, depending on your barns style, you probably won't stay on the same horse all the time. Another thing to remember is that group lessons aren't bad for you unless your instructor doesn't like to take the time to individually critique you.. they teach you how to space yourself out and really control your horse so they don't fallow everyone else. I've been riding for five years & I have only ridden in 2 private lessons. They might be a bit harder to ride in, so if you're just learning, I think one-on-one might be best for you for a little while, but group lessons are fun too.
If you hear good things about the barn, keep it in mind but keep looking! If you go to a barn and you like it, I'd say it's a better choose than the one your friends say are good, ect.