Ther's really no way of acurately finding the snake's age simply by size, because there are so many factors that affect their growth. I have a 7 year old female who was starved for the first 6 years of her life. She is tiny compared to my other adult BPs. I also have 2 very healthy 3 yaer olds who appaer to be dwarfs or something, because they just never grew, despite eating kike little maniacs! So, there is really no telling. As long as your snake is healthy and growing at an acceptable rate, it doesn't really matter that much I guess.
Health and happiness to you and your snake!
-----
GratefulFred