No set age for fanning, but my impression is that small hatchlings would be less apt to do it than larger turtles. The first time I saw my RES do it, he was about the same size as yours
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-
Fanning is when a male turtle extends his penis. It can be rather startling as it is larger and darker than most people expect. It usually only stays out for a few seconds then goes back inside.