Girls football teams are categorised by the relevant age group, with the "Under" commonly abbreviated to "U", e.g. U10 . This relates to the maximum age at the 31st August immediately before the start of the relevant season. Players can be within a two year age band.
FA rules do not permit children under the age of 6 years old to play in matches of any kind.
Players who are aged under 15 at 31st August are also not permitted to play in a match where any other player is older or younger by two years or more than that person.