[Kumagoro]

I've had a look on Wikipedia and all it mentions is risks like infection and allergic reaction, but I'm not sure about the stretching thing. It sounds logical though, so I would suggest getting your tattoo when you've stopped growing. That might be now or it might be when you're 25 - it varies for different people. I recommend you work on your arm muscles first and then get the tattoo; that way you won't risk stretching and ruining the design.