Playing up is a joke.
A 2008 January birthday “playing up” can be ONE day older than a December 2007 kid.
We had a dad going on and on about how is son was playing up an entire year. The kid was a January birthday

.
That December kid has essentially been playing up in his own age group. JFC
The only time it should be allowed is if the kid is one of the top players in the older age group.
We have had play ups that really needed to go back down. These kids could not keep up in the puberty years. I’m all for it if they are a superstar. Same with girls playing with boys. In all my years in soccer, I only saw one girl who truly needed to play with the boys. There are enough girls playing soccer and highly competitive teams that a girls can find a team at appropriate level in her own gender.