Hand to eye co-ordination and innate aptitude tends to be at it's highest at age 16-17 and declines at a standard rate to age 28 where it then remains constant for many year - until the mid 50's early 60s (the performance range is wider then). However, flying isn't all about aptitude, it's also about the quality of decision making which comes through life experience, knowledge and 'maturity' and it is low at 16 and high by the late 20s where it tends to stabilise (it's harder to measure empirically). The intersection of these two rising and declining elements of overall aptitude comes at about 23 years old, plus or minus a year each way, so that's, for the 'average' applicant, the ideal time to recruit. That doesn't quite fit with university graduation, when people are in alife position to apply, but it's close enough.
The younger you recruit a high aptitude person the more risk they don't develop the maturity and decision making, the older you recruit an excellent decision maker, the more risk they don't have the aptitude for some of the more advanced handling requirements.