Mentally adulthood seems to start at about 28
With me it was 17 in Basic, if you take a gander at the avg age of the folks on Aircraft Carriers, towing around 50 - 80 Million Dollar Aircraft, Launching and Receiving 24/7 (the most Dangerous Job in America) these Americans are average age 19, (it really freaked out the Socialist Russian Generals & Admirals when we gave them tours, they thought we were pulling their collective legs)... Put the responsibility on the shoulders, be there for guidance and council.
Hell an Adult can be a 13 yr old without guidance, I got my PADI International SCUBA card from a kid that was 13 years old, he was an American Kid living in the Philippines, he had an Office, Shop, Vehicles, Secretaries, etc. and he was as far as I know the youngest Certified PADI Instructor in the World.
Without confronting reality, you can spend an entire life as a child, I give you Ted Kennedy as an example.
Read 11 and 16. I should have aded it to my first post.