Good question. I have no idea. We didn't do this at home, but my husband and I kiss our son on the mouth. I guess we'll continue to do so until he decides he doesn't want his parents embarrassing him, LOL.
I don't think there's an age limit. One of my kids has always kissed me on the mouth and he's 6 and has finally given it up. My other kids have never done it. I think it's really up to the child and what they're comfortable with.