Kit Darby and his actuarial wizards answered this exact question about 10 years ago. For a person on active duty and going to a major, you would come out ahead if you left the military up to the 18 year point. This was based, however, on the pre-9/11 contracts, including very generous airline A and B funds. At most airlines, the pay and retirement benefits have decreased or been eliminated.
In today's world, it would depend on so many variables (which airline), and now age 65 has added another twist, so the break even point is probably several years earlier. A wild guess would be the 12-14 year point.
If you can get out of active duty, and land a job at UPS, FEDEX, or SWA in the next 2-3 years, then it may be worth it. Getting a Reserve or Guard job if you get out will lessen the loss, as you will start getting the reduced (compared to AD) pension at 60.
Quality of life, for me, is the most important factor. Money comes and goes, but QOL is what brings a smile to my face (along with hot chicks)
Good Luck!