Non-craft parts of flight training can start anytime so long as the information is presented properly. I've taught the basics of things like lift, drag, etc., to kids as young as 7 or 8 (not in an official CFI capacity), and they pick up on things. The more you can acclimate someone to the topic, starting with basic knowledge first, the better prepared they will be when it comes time to take the written, and then to start the actual flight portion of their training.
My daughter is was 14 when she started her actual flight portion of her training. As long as you can reach the controls, it's never too early. Just remember that you have a limited time from taking the written to complete the practical flight exam from a DE, and you can't fly solo until you're 16, so.... those are really the only limiting factors.
One should gauge interest, and how seriously the youngster is able to take the topic of aviation. Kids tend to think they are invincible, and take great risks with physical activities, including driving, so it is not every child that has the mental temperment to be a safe, serious (by that I mean taking the topic seriously and knowing it can be deadly if not treated with respect), and thoughtful candidate. Judgement can be taught, but one also has to have the basic knack for reasoning things out logically and not impulsively.
Hope this is of assistance
