The FA have the following rules in place
1) Under 8’s and Competitive Football
To play in a knockout cup game or a game where points are awarded, or results collected, a player must have achieved the age of 8 on or before 31st August. This also applies to summer tournaments.
In simple terms under 8’s cannot play in games where results are collected and or published. Clubs cannot publish these results on their websites.
2) Children permitted to play mini soccer only
A child who has not attained nor will have attained the age of ten as at midnight on 31st August in a playing season shall not play, and shall not be permitted or encouraged to play, in a match between sides of more than seven
players in that playing season. They are only allowed to play in accordance with the rules of mini soccer.
Players who would normally play under 10 football cannot play 11 v 11 or 9 v 9.
3) Players playing above their age group
Players are not permitted to play in matches where any other player is older or younger by 2 years than that person subject to meeting the criteria of the above rules. Players playing in under 18 football must be 15 by 31st August in a playing season.
4) Mixed football
Up to the age of 11 a child may play in matches involving both boys and girls subject to meeting the criteria of the above rules
In addition leagues can apply for dispensation for mixed football up to the age of 14 subject to an application to the FA. This must be re-applied for at the start of each season.
5) Open age football
To play in open age football a player must have achieved the age of 16.
Kind Permission of TheFA.com