Is it ok to wear this 15 y/o boy?

It would be a good undershirt. If you are wondering whether or not you should wear it alone, I think it would depend on the situation. It would be good for exercising, but for everyday wear and school, it's probably too casual. As a skinny guy, try wearing a short sleeve, on up, print shirt over a white shirt. It's a pretty common style, and it adds dimension.