This is going to sound horribly sexist of me and I realize my feelings perhaps paint a broad brush but....
I wouldn't feel comfortable leaving an newborn/infant with a young male. Not only do most boys lack the maturity to deal with a newborn/infant, they also tend to be more aggressive and less nurturing than a young female.
This is not to say NO boy should ever be left with a small child. I'm sure there are exceptional children out there who are magnificent with babysitting children regardless of gender. I just feel most young females have more nurturing instincts towards children, especially brand new babies. Yes, there are horror stories out there of young females doing the unthinkable as well. But, historically babysitters are often and mostly female.
That being said I realize there are probably many of you here who have had their sons watch their siblings and such. I truly don't mean to imply anything other than what I would feel comfortable with. As a stay at home Mom with very young children I have no experience with this as of yet.