Age decides most of it. A conventional tank in Los Angeles County lasts eight to twelve years, and less than that where the water is hard, which covers most of the foothills, the Crescenta Valley and the Santa Clarita Valley. Under eight years, a repair usually makes sense. Past ten, money spent on a repair is money spent twice.
Leaking from the tank body itself is the end of the conversation. That’s corrosion through the steel, and it cannot be patched.