They choose 18 because of the generalization that the law spawned. Every time a 17 year old cries foul, a bunch of 19 year olds just say "i more matur the u bcuz i 18". Since it's the same everywhere it's not like they can go somewhere else for support. They are a minority and are being discriminated against.

Any other age has the same problem, it's irrelevant. Age =/ maturity.