You can be any age. But if you are younger than 18, you have to have a parent or legal guardian to sign for it.
In the UK it is 18 regardless of parental consent. If a tattoo artist is found to have tattooed a minor s/he may be charged under 'assaulting a minor' and worse case scenario s/he will be placed on the register for offences against children
It depends. In most places it's normally 18, however in some it's 16 and in others it could be 21!