I didn't do this, but I've heard this suggestion if you're really worried about your child having a severe allergic reaction: Take a small amount of PB, smear it on your child's skin, and cover it with a bandaid. If the skin turns pink/irritable/rashy (and not from the bandaid) then your child is likely allergic and could have a fairly serious reaction if he/she ingested it. Obviously, this isn't fool proof, and if your DC is only mildly allergic, it might not work. But if a child who would otherwise have a severe reaction when they ate it, then their skin would also react.
Now for us- I had planned on waiting till they were two, but then all the reports started coming out that thought AVOIDING PB products were a possible cause in the rise of allergies, I went ahead and gave them some a couple months ago. (So maybe 21months old?) We don't have PB allergies in the family, and thus far they've done just fine with it. Prior to that I had been giving them sandwiches made with Sunbutter (from sunflower seeds.) I think the SB is much thicker than the PB, and they've done just fine w/ SB sandwiches for gosh, I'm not sure how long... at least a year.