IS there a stated age? In some countries were some tribes, they tattoo there young at a very age, even in ancient times. But in most countries today. People will have Tattoo's around 16 onwards. As a Christian I think we need to have right decision when having a Tattoo. As we not under law but under grace. So if we as a Christian is to have a tattoo, then we its good to take note to have right discernment of what type. Longs the tattoo don't promote violence, evilness, demonic, even images of Christ. As we all know it says not to create an images from heaven above or the earth below.
Personally I don't think its bad to have a tattoo, and it's not a sin. Just we as Christians need to think and have right discernment as to what to have.