There is no real accurate way to estimate an unknown age.
Ball pythons all grow at different rates to different sizes. For example, I have a 6 month old female as large as a year and a half old male. Some snakes never grow larger than 900 grams, while others are 3000 gram whoppers.
As long as she's healthy, you're good. Age is largely not determinable unless you have a date of birth. Unlike dogs and trees you can't check their teeth or the rings around their trunk, respectively.