Tangs live a long time. 15+ years at least. Mine started out as small ones over 8 years ago and are still growing. If yours only lasted a few years, something was wrong. I feed mine twice daily NLS spectrum pellets, mysis, and nori. They eat A LOT. A pinch of food daily isn't going to cut it.
A starving fish is an immune compromised fish. Keep them fat and healthy and they will live a long time. But with that extra food comes extra pollution which means more water changes and nutrient control.
I think I've read in the past where you didn't do a lot of water changes (apologies if that wasn't you). But clean water is your friend. Not sure if you are keeping corals or just fish, but corals are usually a good indicator of water quality. Fish only systems can go a while with poor water quality because the fish are pretty resilient. Then one day you lose a fish and don't understand why. It's just as important to monitor water quality and do your weekly water changes in a fish only system as it is in a mixed reef.