USMCflyer - just checked. Regs require an ATP. Years ago you could get your ATP but couldn't exercise the privileges until you turned 23.
New standard would allow 21 yrs old w/1,000 hrs if you came from an acredited school. Hasn't passed yet. If it was my son I'd recommend going that route in case it does pass. That way he'd be able to start 121 flying on his 21st birthday.
Seniority, and getting it at the youngest age possible, is everything.
Restricted ATP -
Be at least 21 years old, hold a commercial pilot certificate with instrument rating, successfully complete a new ATP Certification Training Program, pass ATP knowledge and practical tests, and for military pilots, have a minimum of 750 hours total time as a pilot, or for a graduate of an aviation degree program, have a minimum of 1,000 hours total time as a pilot.
http://www.faa.gov/regulations_polic...0-AJ67NPRM.pdf