That is a good idea, and even some 14 yr olds can be 'immature'. I wouldn't leave my little children with 10 & 13 yr old boys either, or girls for that matter.. Here in my area, though, it's quite common. Many 11 & 12 yr olds, mostly girls, routinely babysit for even young babies. Many of them take a 'babysitters' course offered through some local group.
I have an 11 year old grand daughter who I would trust baby sitting a baby for short periods, on the other hand I have an 18 year old grand daughter I would not trust. It all depends on the maturity of the kid.