18. It's an offence to tattoo anyone under 18 anywhere in the UK, even with a parent's permission. The only exception is a tattoo done for medical reasons by a qualified doctor, or by someone working under their direction.
Where you live matters
England, Wales and Scotland
Tattooing of Minors Act 1969
18 and over
It's an offence to tattoo someone under 18, except for medical reasons by a doctor or someone working under their direction. A tattooist has a defence if they had good reason to believe the person was 18 or over, and did believe it.
Northern Ireland
Tattooing of Minors (NI) Order 1979
18 and over
Northern Ireland has its own law with the same age, the same medical exception and the same defence. Parental permission isn't an exception.
What the law says
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 practitioner 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.