You acknowledge that our cultures are different. Why are they different? They developed independently of one another for hundreds of years. Significant cultural exchange between the east and west is still a relatively new thing. Naturally our cultures would exhibit differing preferences in a number of areas.

Personally, I think the western market's preference for "tough" or "bad ass" lead characters is less about an absolute aversion to young, effeminate teenage characters and more about, as JKTrix mentioned, believability. More of then than not, the lead character is thrust into a role in which he must defend the world. This often requires great acts of physical strength and endurance. It's just much easier to believe that Master Chief, a genetically and physically enhanced super soldier is going to lead humanity's campaign to defeat a conglomerate of vicious aliens than it is to believe that Vaan the seventeen-year-old street rat is going to picking up a great axe and slaying dragons and demi-gods.