Where I live we actually have a decent mix of walking and riding. I am 43. I ride due to my back. Granted I never did really walk courses much as living in PA we also have a lot of elevation changes as well. That also plays a factor when walking in my opinion as well as weather and temps as has already been stated.
A lot of courses do not allow golfers to walk on weekends around here to keep pace of play up as well.
All that being said.....I always opt to be the passenger so I can throw my clubs on the cart, hit my tee shot, ride to it then grab what I need and walk the rest of the way from there and finish the hole while who I'm playing with takes the cart and we meet up at the green.
Mike