Probably shouldn't let him, but my 5 year old who is a big Nintendo player is pretty good. He watched me play for along time. At first I only allowed him to walk around maps with no bots. After awhile I let him play what he wanted. Just recently picked up Q3A and he kicks on it too.
Before he really started playing, I was joking with him and asked who was better at UT, me or Mom (who doesn't play, this was a who's your buddy question). He said I (Dad)was better. I asked who do you think would be better, me or you. He looked me up and down, thought a few seconds and finally said he would be better. Not what I expected. Someday we'll have to find out.