The age of the child will depend on what format they can play.

As you can see from the chart children can play up one year but not down.

There are maximums and minimums dictated by the FA. It give you flexibility to pick and choose the format suitable for your team. There are recommended goal sizes which also give you some license.

Personally I think that U13/14 should play on an intermediate pitch with intermediate goals. Full size pitches with full sizes goal can be daunting.

At other age groups you can be flexible. It normally becomes more rigid once team are involved in league fixtures for points!

Mark