The fact is these guys grow so slowly and so differently, there is no age to size mark. More then likely it is over 12 months in age, but it very well could be older. But more important is the weight of the gecko, 2 geckos of the same exact length can vary greatly in weight.
There's honestly no way to tell just by their size or weight. Geckos kept from 70-72 will grow slower compared to ones that were kept at 78 degrees. It all depends on diet, how often they eat, temperature, and just genetics. I had a youngling that grew from 2 grams to 6 after a year. Yet at the same temperature and diet, I had one go from 4 grams to 36 within a year. The first one came from a rescue female that was bred too young, and never took a liking to crickets. The second one came from healthy geckos and loved crickets, and licked their CGD clean every time.
Your best bet would be to get a scale, and keep track every month how much weight it gains. Then you can get a very rough estimate of how old it is (say it's 20 grams and gains 2 grams a month, then it might be 10 months, unless you know for sure you've had it for longer, then you have to make a really rough estimate from there.)