Yeah, I agree, it depends on the maturity of the 16 year olds.
My Godparents kids are 16 and 17 and I would happily leave my 13 month old with either of them. Even though we don't see them very often, he was very happy to be left with them at the back end of the house while we chatted. They love him and I felt very confident with them. I haven't used them to babysit before but would be happy if I couldn't get my sisters for some reason I would ring them.