You can't accurately guess age for reptiles. They all grow differently based on the conditions they were raised in. I've seen underfed snakes be that size at a year old even. The only way you'd know for sure is ask for the breeder's records.
She's likely less than 6 months old (and possibly only a couple months old), but as the above reply says, it's hard to say for sure because there are so many factors that go into their growth rate.