Many books suggest mid to upper teens for typical lifespan for corn snakes, so 15 isn't bad, certainly. Mine have tended to push 20 & beyond. But egg-binding is seriously bad.
"Your" corn snake may well have been over 15- it's impossible to tell a snake's age from looks (size) alone; I'm sure the vet was basing his guess on her size.
Even a very experienced vet might not have been able to save her- surgery is always risky, & can end up with infections too. Not everyone would even put an "older" snake thru a surgery- in balancing the potential healthy years left against the risks, & pain & suffering during recovery (need for antibiotic injections included), so there's that also. Depending on her age, euthanasia might have been a kinder option.