I also wish I started earlier with hearing protection, I guess I was around 22 when I started using it, and I started drumming at 8.... So yeah, quite a few years without, and I start to feel it quite a bit now, especially my left ear is quite reduced.
Otherwise, I wished I practiced more and increased my focus to become better, other than just playing with a band, during my teenage years and twenties. And maybe would have done more towards working in the music business/trying out for a bigger band, etc. At least dare to try, I think I held back for a long time, thinking I wasn't good enough, or I just had low self esteem in general, I guess. At that time I could easily have practiced more, basically no obligations at all. I feel that I now have the skill and endurance I wished I did have back then, but getting close to 40, I feel like I have lost a lot of "momentum", and have more obligations than ever before, so it's hard to be much more than an occational weekend warrior at this point. I guess it sounds stupid to all the 50-60 plus (age) members here, but when playing metal as I do (no other styles of music really affects me the same way), I feel you have the biggest chance of "making it" (or at least getting a name for yourself) if you really work towards it in your 20's. I guess it helps playing in a band with people that all are about 10 years older than me though. I guess age is mostly only a number as long as the music and performance is good, I just somehow wished I could have done it at this level earlier....