I'm also new, but welcome! You are young yet!!!!
I'm 60 and just took up my backpack love again in 2004. Going strong.
My bad back was aided by a good backpack with good hipbelt, properly fitted to me. Feels like heaven to wear. Keeping the core body in shape is crucial to the back. Back muscles, abdominals, laterals, need exercise. You don't need a gym necessarily, just find some good exercises to do for strength and flexibility. Lots of walking!
I agree with earlier post, too, that I would make certain via doctor that my back doesn't have a malady that requires medical attention. I thought mine might, but found I had just gotten soft. I use a gym and walk a lot.
This thing is fun, backpacking. I'm into gadgets, so what can I say - heaven! You don't have to be though, and it isn't that expensive either. Start by walking some trails, stay out most of a day, move to an overnighter, relax and enjoy. You know what they say, just do it!