There is no absolute age for beginning contact lenses. Contacts are used to correct refractive errors that usually can also be corrected by glasses. Glasses are less threatening to the health of your eye so they are usually prescribed first. Contacts are mostly a convenience for patients who do not like wearing glasses.
Because contacts are more expensive and could cause infections, inflammations, scarring or even the loss of an eye, most eye doctors are more comfortable prescribing glasses until they are confident that a teen can be responsible enough to use contacts. I believe that all contact lens wearers must have a good pair of glasses so that they have an alternative if there begins a problem with the contacts. No alternative leads to overwearing the contacts and commonly the onset of infections and corneal scarring.