I'm in my early 20s but I had major back pain for about 6 months. I thought it was caused by my limited flexibility and trying to lift too much weight at the gym without maintaining proper form. My lazy posture didn't help the healing process, of course.

These are easy actions that I think helped me:

-Using a standing desk is probably ideal, but sitting with good alignment is better than bad. I put reams of paper under my monitor to raise it up and I really like this chair (often on sale for under $100): http://www.officedepot.com/a/products/604924/WorkPro-1000-Se... This chair is rated at 8+ hrs/day, and locks to support a range of spinal angles such as this "optimal slouch": http://news.bbc.co.uk/2/hi/6187080.stm

-Be careful with compound weightlifting like squats, kettlebells, deadlifts etc. Using bad form (curved back) will increase the force on the edges of your spinal disks by a factor of 2-3. Trying to lift too much weight compounds this. Both lead to injury.

-Make flexibility a priority over cardio or strength. Do back/hamstring stretches a few times a day until you reach a good level of mobility. I've had the "upward facing dog" yoga pose recommended among others. The therapist I saw also recommended doing "neural glide stretching" where you push for a few seconds, release slightly, then repeat.