That is why the US armed forces has a rep for being so young. Other countries they dont kick people out so easily, but promotions are also much more difficult. Talk to a canadian or a brit. It is not unussual for them to be in 30+ years. And i've met canadian army captains who have been captains for 20 years, giving them more experiance than most american LTCs.
Well, yeah, not a lot of people are going to continue much past 20 years when that qualifies for immediate retirement pay and a private job can suppliment. There's up or out policies at various levels too.