Can someone give me an idea of the age requirement for a guardian?

Robert Adams' Will was proved in Goochland County in June 1740.  

Then I have this from an unknown source and not the original court documents that "Sarah, daughter of Robert, chooses Richard Randolph, Gent, her guardian at the July 1740 Court"

Sarah's mother was still alive.  Why would Sarah have needed a guardian?  Does this mean that she was under age 18 or 16 or some other age?

Thanks
Al Adams

To subscribe, change options, or unsubscribe, please see the instructions at
http://listlva.lib.va.us/archives/va-roots.html