Dear Julie,  

17 is the minimum age for starting the canonical Novitiate in any religious institute. Usually age is not the only factor, so when you get in touch with them they will be take into consideration as well your physical and psychological health, your maturity considering your age, if you are inquiring and want to enter of your own free will, and the studies you have completed (usually at least the 12th grade). 

There are some groups that have a pre-novitiate program where you can do your high school while living a structured life that will prepare you for the novitiate, but you cannot formally enter until the proper age.  

If a young woman proves satisfactory in the areas mentioned above, and is seeking entry for a proper motive, there are some communities that will accept her straight out of high school, while others will require that she get a college degree first. You have to ask.