United Kingdom

Tattooing Of Minors Act 1969

It shall be an offence to tattoo a person under the age of eighteen except when the tattoo is performed for medical reasons by a duly qualified medical practioner or by a person working under his direction, but it shall be a defence for a person charged to show that at the time the tattoo was performed he had reasonable cause to believe that the person tattooed was of or over the age of eighteen and did in fact so believe.

United States

Most states require you to be an adult over the age of 17 although some states allow a parent to given consent to minors being tattooed.

The following states allow with written consent from legal parents or guardians.
Alabama, Connecticut, Delaware, Hawaii, Indiana, Maryland, Michigan, Missouri, Montana, Nevada

The following states allow tattoo on children aged 16 or over with written consent from legal parents or guardians.
Florida 

The following states allow tattoo on children aged 11 or over with written consent from legal parents or guardians.
Idaho 

The following states allow with a legal parent or guardian physically present.
Arizona, Arkansas, Kansas, Kentucky, Louisiana, New Jersey

No written rules, artists own rules apply.
Colorado