What is a good starter bike for a 11-13 year old?

I would assume you are refering to dirt bikes. I have recently bought a 80cc for my thirteen year old son. It works quite well for him, other then the fact that he will be growing out of it soon. I have also recently bought a 100 cc. This seems to work well for him, too. The 100 cc has plenty of power, yet not too much for him, yet plenty of height for him. My 11 year old son rides the 100cc as well. He has recently started out riding,too. Hope this helps, Ben