Under the age of 15: Back problem? There IS no back problem!
Age 16 to age 18: Nice horsey tossed me off twice in a row trying to get over a fence. Solution? A couple days in bed, and two years riding hunters solely in two point position-- no sitting trot, and NO western.
18- 21: Riding all sorts again, but still staying mostly English to save my back, and getting picky about what trots I sit. "Son, they can call you a pleasure horse, but if that's all the smoother you go, forget it." Lots of sit ups and leg lifts, along with stretching exercises.
22-24: Stupid western saddles don't fit anymore. Took me two years to find out I needed to be in a size 16" western seat. Even then, most saddles weren't comfortable.
25 - 32: If it's a Circle Y 16" equitation seat, I can ride it. If it isn't, don't even ask me to stay in it for more than an hour. English saddles still fine, but a few added falls and a baby mean that I'm more careful what I ride, and I'm not starting any rank babies anymore-- they hurt when they blow up. Meds for my polycystic ovarian syndrome somehow make my back hurt less-- wish I had known THAT ten years ago. or Fifteen, even.
32-36: Sticking with Circle Y saddles, maintaining my back exercises, and bought a new bed mattress (yea, much more comfy sleep). Time constraints and the fact that I divorced result in horses that don't really do much in English, so I'm mostly western. Probably just as well, since I've put on pounds now.
In the last year: I switched to gaited horses for something new. I don't know that they are really that much smoother (my gelding can really bounce me when he messes up his canter), but they sure are fun. To go with them I bought Crestridge saddles-- a Guffey (comfy big time!), a Crusade Lite( Also comfy!) and an Ovation (Nice, seat is a little narrow for me though). These are my trail saddles now, to go with my trail horses. Also, when I ride trails, I don't walk the whole time, because it really kills my seat and back. I have to mix it up, do some gaiting, some cantering, back to a walk, and so on. It's only a problem on longer trail rides, over an hour.
Also, I've seen people recommend Tucker saddles for trail riding. I've also personally known people who bought them and hated them. All those people are, without exception, NEW Tucker owners. The company was bought out a couple of years ago, and the same maker isn't making the saddles. Evidently the quality is going down because of that. The same could be said for new Circle Y saddles-- the original makers of Circle Y sold out, and now they make Reinsman saddles.
It's not horridly important, but these little industry shifts are good to keep in mind if you decide to saddle shop.
Also, you didn't specify what discipline you ride in right now, but I honestly can't do formal dressage-- those deep sitting trots kill my back, even to this day. I still love and respect dressage, and I use the techniques often, but no more actual showing or fitting a horse for show.