Given that Lemmy is largely populated by programmers, I assume they have done next to no exercise through their 20s and mostly sit hunched over in a chair all day. So now that they are hitting their 30s, they are paying the price for abusing their bodies.

You don't need to be huge like Arnold, but find some way to move your body every day. While you're young in your teens, 20s, and 30s find different and interesting ways to keep your body moving and build up strength, whether that be swimming, running, larping, lifting weights, finding a local exercise meet-up, local sports league, or walking to a park and using the little exercise machines.

A lot of the back issues will go away with more core strength and lifting things properly with your legs. If you can't think of anything, then start by sitting up straight and tall in your chair using your core to stabilize yourself and see whether in a few weeks you can get through a half or maybe even full day without slouching. Anything is better than nothing!