For DD9 it was when her wonderful first-grade teachers started giving her math lessons with the older kids (it was a Montessori classroom for 1st-3rd grade). Later the other kids started making comments to her (not mean though). Now she's pretty much fully aware of it, although we still haven't shared everything about the 2e stuff because we're still not sure what the answer is. But I think letting her sort it out worked out pretty well. Plus I don't know what you could tell a kid that age ahead of time that they would be able to understand and process in a sufficiently mature and useful fashion (although I'm sure there are some that can--but not our DD).
