You do realize that there is no group who is more in love with superficial action and weapons than young teens, right? That's the age when they obsess over silly martial arts movies, gun fights and weapons. Seriously, who didn't play with nunchucks, throwing stars and gun replicas at that age?
Black Lagoon, Bleach and such shows are just an extension of that.>>13222032
I don't really care whether a show is mature or not, I just don't like exceptionally poor shows like Black Lagoon. That also goes for most Hollywood movies that use the same clichès.
As for "mature", I guess I can mention the Kenshin OVA, Ghost in the Shell and Jin-roh.