I can't remember... I think my DH had it when he was under 2... his older siblings had it, and it was a mild case for him. What I've heard is (this was before the CP vax) that the younger, the better... i.e. 2 or 3 is OK.
But caution... a friend tried exposing her daughter throughout her young life. Her DD finally got it at 12/13 (or so) just as the cusp of puberty... and it was
hellish. She was it inside her vagina, inside ever where! Her DD was mad at her mother for years. The DD is now 21.
My son is 3.5. A few months ago a friend had it. The 4.5 yr old had it and the 1.5 yr old had a much milder infection. I wanted to expose my son, but my DH wimped out. He felt our son wouldn't be able to handle it "don't scratch." Well, he would if it wasn't such a bad infection, kwim?
Anyway, a friend sent her DD (same age) over, but she didn't catch it.