The age issue depends entirely on the individual horse, how healthy they are, what injuries they've had in the past, how hard they've been used, etc. I know a horse that is 18 and quite old. But my friend's thoroughbred is 30 years old and still goes on 2 hr. Trail rides that include lots of galloping. This friend is a real horseman who would never push a horse, but this mare has been kept fit for the past 10 years, and she is an ex-racehorse who would rather die than be left behind while other horses go out on a trail ride. She is like the sled dogs that want to die in the traces.
That being said, the key is "every day." If you only ride every couple of weeks, then trotting for an hour is excessive. Also, if you ride every day without a break, that can be wearing on any horse, young or old. They need recovery days for their muscles just like human athletes do.
Eolith's points about feeding are very important. A lot of younger riders increase their horses' exercise without significantly increasing a horse's feed and the horses lose weight and condition. Your horse will need extra food if she is working out more than she used to or she won't be able to build muscle.
Finally, just be sure that not only you are having fun but that your horse is enjoying herself too. Doing one thing over and over is boring for any horse, and repetitive cueing can make a horse sour about working. In your video there is a lot of tail swishing going on. Yet your horse seems willing to try to figure out whatever it is you want her to do, so be sure you reward that great attitude with lots of breaks and rewards.