Its not so much an age thing, it's mainly a shell length thing. At 10-12 inches the sex is fairly easy to say but some males aren't "obviously" males until 13-14 inches. My male was noticable at 10 inches but my buddies male wasn't obvious until 13 inches because his has much a different anal scute shape. Some females have incredibly small tails at 8-10 inches to where being a male would be a stretch. Yours is a classic example of being right in the middle. It's tail is rather small from what I can see but the anal scute shape could end up going either way. Some people attempt to sell "so called" young females that could very well end up being males.
Give it another good year of growth and it the sex may become apparent.