I started at 50. Riding uses muscles I hadn't used in jogging and other sports.
I think riding western IS easier on the body than riding with a forward seat. I can't speak to riding English in a dressage seat, since I haven't tried that.
I found riding in a "chair seat" easier on my legs than trying to put my heels under my hip, although the books all said I NEEDED to get my heel under my hip. Unhappily, many western riding instructors have followed the 'heels under hip' stuff, which isn't required for good western riding.
One of the problems with starting when older is that many riding instructors don't know what it is like to start with an older body. Many learned when young and flexible, and they don't understand the trade-offs that need to be made when you aren't a teen.
You also don't need to post with western riding. You can if you wish, but it is entirely OK to just sit in the saddle during a trot, and bounce a bit until your subconscious mind learns how to minimize the bounce.
If the western horse knows how to neck rein, you can ride him one handed. I find it much harder to be tense when I'm riding with one hand resting on my thigh.
As you do more riding, gaining both skill & flexibility, you can THEN adjust your style of riding - if you wish. A lot depends on your goals.
If you want to jump or perform in dressage, you would need to change your style. If you wish to ride trails, then you can ride western with long legs, a chair seat & one-handed as long as you wish. The cowboys around 1900 did... Erwin E. Smith Collection Guide | Collection Guide
I suppose I ought to add that the six-gun is optional. It would make some riding instructors a bit more polite, tho...