I think that it is largely genetic, but it generally
is easier for older people to be morning people than
younger ones.
Motivation is a large factor too. I became a morning runner,
late in life, after having a couple of different people in
my neighborhood trying to get me to stop, in the middle of a
run, and chat with them.
I have awaken no later than 4AM for the past couple of years
and run every morning, unless injured. Thirty plus years ago, my first class at a large university was after 1PM and I often
slept until 10 or 11PM.
All that said, I do a fair amount of doubles now, and my PM
runs tend to be a bit faster with equal effort.