Life spans can vary greatly, and for many reasons.. even the care they receive as fry/juvies can result in shorter/longer life spans.
Water quality and food quality are the top two in helping to keep them healthy and live longer - even with poor genes, those two things will help keep them healthier. Even still, some are more susceptible to illnesses than others regardless how clean you keep your tank.
One thing to keep in mind is a tank can be too clean - there is a balance that you need to keep in order for the fish to be exposed to certain bacteria and such in order for them to gain resistances to illnesses. Weekly water changes are a must, but it depends on tank size, filter, cycle, tank mates, live plants, etc. Oldfishlady has a wonderful guide
that will help you find what works best with your set up.
With using AQ salt, just "throwing some in" won't work - over exposure will cause the bacteria to become resistant to it. Once you are definitely sure there is fin rot, you will want to do the recommended treatment for it. Over exposure of AQ salt (using when not needed) will cause resistance to it, along with possibility of causing kidney/liver damage. Best to make sure he has it for sure and then treat accordingly. Seeing shredded/missing fins normally doesn't mean fin rot. And some report AQ salt can hamper new fin growth.. best to just make sure to keep up on the water changes and feed high protein food to help growth/healing of fins.
It depends on how large your tank is whether or not the 50% weekly is appropriate for you.
So a mix of genes, care given at birth, water and food quality will decide how long your betta lives. There really is no secret, just proper care.