I think it's swings and roundabouts really.
Young guys have no trouble getting hard and cumming fairly easily, can often go twice (or more) in a booking
Older guys can have difficulty getting and staying hard, and may take a lot of work to cum, and then that's it.
But..
Young guys often have no clue about how to get you off (or any desire to) and sometimes want to emulate some ridiculous shit they've seen in porn
Older guys can be much more skilled giving oral and more realistic about how long it will take to get you off.
Today I've had 4 bookings and it's been 3 older guys and one younger. They've all been polite and punctual.