Cath is right, and 'special needs' covers a huge spectrum from mild dyslexia to a vegetative state.
Not only does the child have to be considered, but also the babysitter. I know one 14 year old that I would trust with any child and a few 18 year olds I wouldn't trust with the care of a cat.
The law does not specify a minimum age for babysitting, so use your common sense.