We need to decide

As a country we need to decide at what age you are an adult and when you receive the rights and responsibility of adulthood. Is it 16, 18 or 21? We seem to draw the line at a different age for various incidents. 16 for driving and consent, 18 to serve in the military, vote, buy cigarettes or long guns or go to adult jail(though they break this one regularly) and 21 to buy alcohol or a handgun.
I believe we need free collage for any qualifying children. First four years in a state collage. If you want to enter the military you can't join until 21 and you will have to have three years of collage finished prior to entrance(your service counts as your fourth year of collage).
We should raise the legal status of "adult" to 21 for everything with the exception of driving an automobile and possibly consent.
What are your thoughts on this?