Parents often wonder what age their child should start karate, or indeed any other form of martial art. In terms of the law, there is no minimum age for anyone to practice karate. Many karate schools do however impose their own minimum age mostly due to their insurance policy which usually requires students to be at least 4 or 5 years old. Even if allowed to train under the age of 4 I would not recommend it except in the rarest of circumstances.
I will now give some general guidelines on how to tell whether your child is ready to learn karate. Of course you know your child better than anyone so any advice I give should be balanced with your personal experience. From what I can see, the two main factors that decide whether a child is ready for karate are whether they are mentally developed enough and have sufficient physical control.
1. Attention span. You should ask yourself whether your child has the ability to concentrate for at least a few minutes at a time. Paying attention is essential as not doing so can result in anything from them not understanding what they're meant to be doing to injuring themselves or others. In addition, many moves have several stages to them so not being able to focus will be a problem.
2. Appropriate manners. Your child will need to have appropriate manners. They should be able to listen quietly to an instructor and if they have any questions they should know when it's an appropriate time to ask them.
3. Empathy. Another essential quality is the ability to empathise with others. More specifically they should be able to understand how their actions affect others.
4. Comfortable without parents. While parents sometimes watch for the first lesson or so, they eventually leave at the beginning and return at the end. Kids should therefore be comfortable training without their parents present.
Really all they need is a sensible degree of control, balance and coordination. Control is necessary to prevent injuries. Balance is needed to be able to maintain stances and stay in position. Coordination is needed to use different body parts in conjunction with one another. Most of these physical attributes will improve with practise.
Karate encouraging violence is a myth. See here for more information.
Really the only way to know whether your child is suited to karate is for them to try a lesson. Karate schools are in almost every town. Look out for them in newspapers, on the internet and on notice boards. Many karate schools offer training at least once per week and some even offer a free first lesson.
Forcing kids to do anything will only make them resent it in the long run. We used to get a whole group of kids who were forced to come to karate. Their parents used to dump them at the karate school and go to the pub for the next hour or so. Forcing kids to go to karate will not work and will just be a waste of money.
It may be worth noting that kids generally commit to karate for longer if they train with a friend.