What would be police do if a few 12 year olds were smoking?? At least stop them or would there be nothing they could do, apart maybe say it's bad, mmmm 'kay??
Section 7(3) of the Children and Young Persons Act 1933 states the following:-
(3) It shall be the duty of a constable and of a park-keeper being in uniform to
seize any tobacco or cigarette papers in the possession of any person apparently under the age of sixteen years whom he finds smoking in any street or public place, and any tobacco or cigarette papers so seized shall be disposed of, if seized by a constable, in such manner as the police authority may direct, and if seized by a park-keeper, in such manner as the authority or person by whom he was appointed may direct.
Cigarettes and Booze are regularly confiscated from groups of youths - the area I work in uses Special Constables and PCSO's under the direction of a couple of Police Officers to patrol hotspots for nuisance youths - you just wouldn't believe the amount seized! (and no it doesn't go to the Staff annual party - it gets destroyed).
There isn't as far as I am aware any law that says "it is an offence for any person under the age of X to smoke", but the above law obviously stops anyone under 16 years of age smoking or posessing tobacco etc in a public place. In otherwords the child isn't going to be prosecuted if they are found in such circumstances.