Is it okay for kids to be gamers?

I'm 14 and I consider myself a hardcore gamer. But when I play Mature games like Call of Duty, there's always some jerk making fun of me because of my age. I find nothing wrong with playing and enjoying games with disregard what the rating is. The rating is "Mature" so doesn't maturity matter more than age? I'm not like the immature 9-13 year-olds saying "noob" and "rofl" all the time. I just want to be considered as a hardcore gamer just like an adult can be. The answers I'll get will probably say I'm too young. I just don't get it... I have a lot of consoles and a PC. I watch E3, I play God of War, (Yeah, I know it has nudity in it. It's really just a stupid mini-game and some in the scenery) and I play many other games too. I asked this question because I'm just wondering why people think that only 18 years or older can be hardcore gamers. I get laughed at when I play GTA IV online. What the heck?