they were always childish. a bunch of giant walking talking turtles i dont see where that isnt childish (besides the weapons).
as for them trying to look cool i watched the turtles back when i was like 6 years old now im 19 and i still think theyre awesome. despite them being childish they still appeal to an older crowd at leats the ones in my generation because all my friends went and saw it and loved the movie.