[Thamnophis]

I have had gartersnakes (sirtalis parietalis and radix) that went over ten years of age without problems.

I am sure that a healthy, in captivity living gartersnake that hibernates every year for a month of three can get 15 to 20 years.

In our reptilezoo we keep a number of animals that are there since we opened in 1982. And at that time they already were adult.

Some examples...

Several Python regius that are at least 29 years old.
Epicrates striatus at least 28 years old.
Gekko gecko (Tokeh) that is around 30 years old.
Some Iguana iguana live for about 18 - 20 years in our zoo.

All these animals are healthy and do well.

This does not happen that much by persons at home.