It depends on how one exhibits the word "Prime" to the opposite sex.
There are women who like younger men, like those without experience. For them, the "innocence" part means that it's the best that the man can be before external influences set it.
There are also women who define "Prime" as being more experienced and worldly, who want the men to take care of them and pamper them, etc etc etc.
However, regardless of the women's preferences, there are baseline requirements that need to be fulfilled prior to being in the category of "prime".
One of them certainly is physical appearance. (yeah yeah, people will say "no, as long as they make me laugh, etc etc" but that is not true)
Another is financial stability. True, you can "present" yourself as being financially stable without actually being one, but that's another story.
Then you have skillsets. Men who can cook (but not necessarily do so), who can play sports, musical instruments, drive stick, do basic car stuff, can hold a decent conversation, etc all these add to the category of "prime"
Hygiene and outward appearance matter as well. I've separated this from the physical appearance because even if you have a six pack, but the only things in your wardrobe are boardshorts and tshirts and you go out to certain places, the sick pack wont matter if you stink or if everyone is on business casual and you're wearing flipflops.
TL;DR: "prime" differs depending who you want to attract, but there are pre-requisites regardless. If you're insisting on limiting it to age, then it's until you're 50( if you're a dude). Of course, the pre-requisites still apply.