Personally, I would imagine a rookie capsuleer in one of the four major political/military/industrial environments to hover around their early 20s age wise. This based on an assumed (which could be wrong) standard training regime that starts in your late teens and goes to 23, 24 years of age. This would incorporate an OCS, as well as a flight school curriculum that aims to get capsuleers skywise not only in a timely fashion, but in a well trained, flight competent manner. One that not only educates in the ways of combat, but in the attitude expected of a respectable capsuleer befitting their position (not that the attitude would stick past school, anyway).
That being said, I also would think that capsuleers would exist outside of that age group. Just as we have older individuals joining today's military, so would there be the same dynamic in the capsuleer corps.
You can also add into the fact, that some, such as pirate and outlaw organizations, could conceivably have older or younger pilots as not only is their technology of a different quality and standard, but their requirements are as well.
Pirates would have little concern for the overall education of the capsuleer, I would imagine, and would place more of an emphasis on the down and dirty skills needed to effectively fly the craft. This could lead to individuals placing outside a standard age group, although I also couldn't see them giving the keys to the corvette to someone still in adolescence.
Which makes me also wonder about the technology aspect. I'm sure the four major groups have different yet equal levels of capsuleer technology, that keeps their pilots on the cutting edge of the operational envelop, but it leads me to think that much like third world countries in today's time, the pirate and outlaw factions, while still possessing adequate tech, do not measure up equipment wise to their 'big brothers.' This could produce an odd strategic and tactical doctrine, as some might focus on training the skill level of the capsuleer to be equal, or greater than their hunters, and some might opt in for the 'blob' mentality, where it's accepted they can't match the skill and tech of their enemies, so they'll overwhelm with sheer numbers and hope for the best.
I suppose this would then lead me to assume that capsuleer age might not be so dependent on where the individual is in their respective growth cycle, but where the person fits into the strategic and tactical system of the employing entity.
The four factions might wish to display a professional appearance and attitude in their capsuleers they feel wouldn't be best served by extremely young individuals, while the more dubious side of the fence might not have the same reluctance.
Just my 2 ISK.