I am curious about how much is actually known about the lifespan of a very healthy garter... I have heard from multiple rather reliable people that they had, or currently have a garter live or living to almost 20 years. Many, many other snakes are able to live to at least 20 and closer to 30, I don't see why it would not be realistic for a very healthy, captive bred thamnophis fed a just-right, well balanced diet that does not contract some sort of illness or parasite!

Sorry to hear of your loss... I have 7 year old kingsnake who was my first pet snake and I love him to pieces, and even though he has MANY years left ahead, I don't like to think about how old he already is!