What is the right age/ability to consider starting Motocross?

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Hi Everyone, I have three boys, my middle son has always been bike mad, peddling around before his elder brother. He is asking for a Motor bike for Xmas (don't think Santa could fit that in his sack) He is too young to start motocross now but I was wondering when is the right age/ability and what is the best way. I always enjoyed watching but have never taken part. Thanks - Lianne