I reckon its about the age by which you had memorised the players names and wore jerseys of your own volition
thats a bit unfair , we grew up Liverpool , there was never any element of choice. Its like you are catholic or prodestant, you can opt out when you are older but you dont choose.
Personally I find it strange when i meet people who 'choose' to support any team. Like I know a kid who supports Arsenal and his dad supports Spurs ! what the fuck kind of fucked up family/parenting is that . We would have been sent to the orphange for wearing an Everton shirt.
As for naming the team my cousin could recite it before she could even speak in sentences, her dad drummed it int her. she was probably 2.
I haven't wore a replica kit since the one i got for xmas in 1976 tho.