Snakes can be sexed at any age. If you have intentions to breed later down the line, buy a pair of neonates. They will grow with you, teaching you about these lovely animals.

From what I have heard, blue striped garters have a tendency to stay on the smaller end. I have been told that the pugets have the potential to get quite large. I would plan to have a 20 gallon tank minimum for one snake, increasing in size with more.