Upper register doesn't sound mature enough
I'm almost 18 years old and I got to a point where I have no problem crossing between registers, I can hit a G5 anytime I want and I rarely struggle singing any type of song (in terms of technique). However, I often avoid singing in my upper register - and songs that imply that - at gigs because on the high notes above G4 (roughly) my voice lacks a certain crisp that would give it some more character. In recordings, it sounds almost like a very loud falsetto, and I can force myself in any way possible (more breath pressure, more air etc) and it still sounds like that.
Is it normal? My voice has indeed always been a bit behind my age (I can tell by listening to old films and recordings in comparison with friends who were even younger). But shouldn't it start sounding more mature? Should I wait and keep singing the way I feel at the moment most comfortable? Or are there any exercises that could help? Any advice would be appreciated.