Yes, Mulv is right. Unfortunately they all grow at different rates (just like humans), but if you give us her weight and length and feeding routine we could have a guess.
Thanks for that I was very unsure when I got her as to their account of her age. Despite being a very good and reputable place. Is that a normal age to sell them then? Any other tips in respect to a Royal of this age would be much appreciated I am absolutly in love with this snake I cannot stop looking at her which is easy as she like to come out and sleep on top of her log/hide. This is the most fascinating animal I have ever kept.
Its very good that you have a Royal feeding regularly on rats already. Guessing the age is very difficult, especially when coming from a pet shop where a lot only feed a 'maintenance' diet which will keep them going but not enough for them to grow very much. So it could be a bit older.