At what age would you use the term "bully" for a kid?

this is not directed at mozz: surely it can fit by middle school. but there's a current discussion on our town listserv about how to deal with kindergarten "bullies" (quotations mine, not theirs, some people are suggesting trying to get the young perpetrators expelled) and i've heard parents use the label for kids even younger than that, including preschoolers and even toddlers. if a 4-year-old is calling your 4-year-old names, do you consider him a bully? if a 5-year-old pushes your 5-year-old at recess? is there some age at which you just call it bad judgment, or say it's a kid who hasn't learned critical social skills yet and needs some interventions to help with that, rather than use the word "bully?" or is bullying still bullying even when done by somebody who can't spell the word yet?