My main riding horses are 19 and 20. Still going strong and if you let them sit, boy they get perky and can still bounce off the walls!
I also have a coming 3 yr old and after all the angst and worry of training a young horse, I personally have decided that young horses are overrated. Why stress and worry and spend tons of money hiring trainers when you can go out and buy a horse you can actually have fun with (and is safe) right away? I LOVE my youngster, but sometimes I wish I could just ride him and relax with no worries the way I do with my older guys. Even if a young horse is behaving, they really don't seem to have auto-pilot like an older horse. You always have to be on your toes.
If I ever came into fortune, I would take in older horses and give them a loving home. Too many of them are under-appreciated and that always makes me sad to see people selling horses that gave them the best years of their life.
I guess that was slightly off-topic. But yes, if a healthy 20 yr old can pack my large butt around for 5 hours at a time on mountain trails, they should be able to carry a younger brother no problem. But healthy and sound is key.