You can do it, if your heart is into it. In my class, we had an older guy that came from Korea. He already had 15 yrs in service, and was an E-7. I think he was something like 40 - 42 years old. Hell, as far as stupid school goes, he may as well have been 80.
It got tough on him though. Winter in Mountain phase ain't no joke, even for younger guys. I didn't think he would make it. But the old codger pulled through. By Florida phase, he was out of gas, and still didn't have his Florida "go". On the last patrol, some of us Bat. Boys volunteered to him to be his SQ LDRs and TM LDRs for his PSG patrol, and we helped him get his go. The Tabs on this board can remember what the last patrol is like, and we didn't want him getting buddy fucked on the last patrol, and have to recycle. In our minds, the old man hard already earned his shit, just by hanging in there like he did.
I think it damn near killed him, at his age. But, he won a lot of respect from a lot of people in doing it.
As far as "on paper" goes, I have no idea what the age cutoff is.
A Co 1/75 '94-'97