The reality in the wild is that approximately 85% of offspring do not survive their first winter. Of the one's that do survive the first winter, only a tiny fraction survive long enough to have offspring of their own, let alone reach 10 years old.

21 years is dang miracle if you ask me, even in captivity. I'm still shocked about that. A lot of people don't even believe me. I have no reason to lie about it. I can hardly believe it myself.

Other garters I've kept lived 6-9 years.