Modern society likes to put pressure on youth to grow up faster, be mature earlier, or act like it anyway.
I can't think of any disadvantages -apart from, maybe, stress- for the moment but I'm pretty sure it's unhealthy to grow up faster than one is supposed to.
And I doubt you can prepare a person for the 'real world' all that much, even if they mature quickly, they still won't have dealt with serious problems and the likes unless they've actually been in the situation because of i.e. their parents. The only real way to prepare someone is through experience, this applies to everything.