There aren't any actual rules that give a minimum age for synchro skaters. However, I would think that 4 is about as young as you could go. Most 3 year olds (and a lot of 4 and 5 year olds) just aren't able to pay attention long enough to learn a synchro program, or remember all the things in it. Most kids that age can't skate well enough to do synchro, either. They have a hard enough time skating by themselves, let alone trying to coordinate their steps with a bunch of other skaters. I'm working with a bunch of kids on a group number for a show right now. It's not a synchro number, but the 4 and 5 year olds in the group are having a really hard time following directions and remembering what comes next.