How old is my Bonsai?

From BonsaiWIKI

For collected trees, it is possible to estimate the age of the specimen by counting the annular rings in a cross-section of wood collected from similar trees in the same area.

Experienced bonsai growers can guess surprisingly well at the age of cultivated specimens based on characteristics like the texture of the bark and the degree of ramification of the branches.

Overall size and shape aren't reliable indicators. If you were to post a picture of your tree on the bonsaiTALK forum, I bet some forum folks could give some educated guesses!