Definitely a red sided. My first (of 2) garter was a self-caught red-sided as well.

It looks to me like a female (the faces are a little bigger, wider etc.) but like Shannon said the tail looks male... hard to tell.

The easiest way to judge the age is to watch his shedding (how often he sheds). If it's once or twice a month you can assume he's with 2-4 years.

Keep us posted, good luck.