As an FTO at my agency, the hardest thing with older people (and I am generalizing here so please no one take offense) is that they are unable to multi-task as intensely or as efficiently as younger officer's seem to. Poorer vision and coordination along with the mental stressor of learning a new task while being evaluated during a time in their life where they might feel they should be the one teaching, can contribute to difficulty in completing an FTO program.
Now, officers are talking on the radio, trying to remember call locations, trying to remember agency policy in how to handle the call and then what they are actaully going to say when they begin talking to the reporting party. They may also be reading instant messages on the MDC, getting updated call info while en-route, and trying to put a perimeter together on a tac frequency with other responding units.
This is overwhelming for most folks, young or old, but like I said, IN GENERAL it appears that the younger a person is that I get, the easier it is for them to process all of this simultaneously. Under stress, "older" people have a much more difficult time staying mentally sharp and on task.
And of COURSE there are plenty of young 20's folks that make it through the process that are straight up retards and should never have been hired in the first place, BUT that's not up to me!
Physical is definitely important, but I believe mental acuity and multi-tasking are higher on the priority list if you want to be an asset and not a burden to your squad/agecy.
And honestly, having worked with many ex-military, (and each one of them a damn fine human being by the way) not a single one of them would deny that life in the military, even as an MP is nothing like being a street cop in a city agency.
My .02