Kantar, you bring up some very interesting points.

1. Wild caught garters can do very well in captivity
2. Indeed babies are the most likely to die from natural causes like snow. However, even babies naturally hibernate in the wild.
3. Human development kills more snakes than pet collectors. Your area sounds like it only supports a fraction of what it used to. This means;
a) Your individual snake has decreased chances of survival in the wild than it would have 10yrs ago; it indeed likely will die (as in all neonates)
b) This stressed population needs all of it's animals to survive and isn't a good candidate for collecting.

Ian