The APD is run like an organization with certain policies and regulations governing it. Psychology proves that younger age equates to less vigilance of your actions and a willingness to do your own thing (a part of maturity). The APD doesn't need that.
Because of the nature of the APD and the large number of whitelisted officers, the APD needs individuals who are able to abide by a piece of text and take an order. Not saying you can't, but generally that is the case around your age group.
And just for the record, the age limit has been lowered considerably twice in t