Id imagine its as simple as older = reliable, courteous, experienced. Younger = cum quick, selfish, hassle.
Or to turn that on its head, Older = Takes longer to get it up, shoots once then cant manage another not as athletic in bed so she takes less of a pounding/doesn't have to work as hard.
Younger= Actually wants a good shag, more stamina, wants more than one pop, she has to work for her money.
Both sweeping generalisations obviously but I bet its fairly accurate as to what they are aiming for when they make these limits. Or she likes the older guys as she has serious daddy issues