It is a hard thing to do, recommending a martial art. The art itself is one thing, but the attitude and quality of the club and instructor/s is also a major factor to consider and look into before choosing a club to join.
There is such a thing as too much disipline, and some clubs take things a little too far and can be too rigid. There has to be an element of humour (in the right place/time) to help you enjoy your training, especially as a Junior. Nothing worse than the thought of a kid growing up and taking themselves too seriously or ending up thinking they're invincible etc..
On the arts themselves, I did Karate from age 12 to 21, and whilst I would say that it was a great thing to do to learn a fantastic martial art which also helped me by teaching respect of others, discipline, control, self defence, fitness etc.., as an adult I decided to try something that had always tickled me fancy - Aikido. I am now wishing I had started this at age 12 rather than Karate. I much prefer the philisophy behind Aiki (specifically Tomiki style Aikido), as well as the art itself which I have found makes you think a hell of a lot about what you're doing and why. Plus the fact the techniques are also much more fun to practise (IMO)!
As for ages, my club accepts kids from age 6 and the training is obviously made safe and tailored to suit age / ability as you would expect.
What area are you in?
Please make sure you shop around 
