Putting legal definitions aside, at what age do you think someone can really be considered an adult?
At whatever point they can be compelled to answer for crimes as an adult, serve in the military, or vote responsibly.
However, the age of drinking or playing with guns should match the age of the above. If that requires one to move up, or the other down, so be it. But this whole "you are an adult, but we don't trust you with X, Y, or Z" is ridiculous.