The standard of care expected of a young child is very low, and even teenagers are not generally expected to meet the same standards as adults unless they are doing adult activities.
McHale v Watson (1966) 115 CLR 199, High Court (Australia)
A 12-year-old boy D threw a home-made dart at a wooden post; it either missed or rebounded, and hit and injured a 9-year-old girl P standing nearby. The appellate court upheld the judge's ruling that D's conduct was to be judged against the standards of the reasonable 12-year-old, and that consequently he had not been negligent.
Mullin v Richards  1 All ER 920, CA
Two 15-year-old schoolgirls P and D had a "sword fight" with plastic rulers in their classroom; one of the rulers snapped and a piece of plastic entered P's eye, causing permanent damage. P's claim against the local authority was dismissed - on the facts, the teacher had not been negligent - but she succeeded against D subject to a 50% reduction for contributory negligence. Allowing D's appeal, the Court of Appeal said there was insufficient evidence that the accident had been foreseeable in what had been no more than a childish game.
Ryan v Hickson (1974) 55 DLR 3d 196, Goodman J (Ontario)
A 9-year-old passenger fell from a snowmobile while turning round to wave and was struck by another through the negligence of the 12- and 14-year-old drivers; the judge said the drivers' knowledge and previous experience made them fully liable, and awarded damages against both the drivers and their respective fathers. The damages were reduced for the contributory negligence of the passenger, however, who in spite of his age had sufficient experience that he should have been able to take better care for his own safety.