Many recommend that girls can use menstrual cups, even at the age of 12.
Just like tampons, it is inserted into the vagina during menstrual cycles. If a girl is old enough to menstruate, then they can use a menstrual cup.
Additionally, Ruby Cups are saying:
“Using a menstrual cup will not take away the virginity status (or stretch out the vagina) of a girl or young woman.”
It is best to inform young girls how menstrual cups work, the benefits, and the insert/removal methods. As long as they are comfortable with the idea of trying a menstrual cup, then it is okay.
Users of Lunette Cup, for example, are as young as 12, and they are already rocking their periods with Lunette menstrual cup.
Another menstrual cup brand, called Mooncup, says that their cups are designed for younger girls, and they know a lot of young girls, from 11 upwards, using their cups.