How old should a girl start shaving?

There is absolutely nothing at all to say a 13 year old girl, nor anyone else of any other gender or age, should shave. Shaving is a personal preference and not something that people have to do. How often a person shaves is also down to personal preference and how fast their hair grows back or why they shave - e.g they may shave more often if their hair grows fast and they shave to wear dresses daily.