A.S.
IMHO, when they are old enough to want it themselves and take care of it themselves...maybe 2nd or 3rd grade. Babies are adorable enough on their own...no need to add accessories, especially ones that involve altering their body.
If you do decide to do it, I would ask your pediatricians office. That is probably the safest place to do it as far as not risking infection on a child so young.