Built in babysitter

it really depends on the maturity of the child, and I feel that it is helping the child to learn responsibility

I think it's hard to put an age on it without knowing the child. It all depends on their individual responsibility. Some 13 year olds are extremely responsible - and others act like 6 years olds. And no - I don't think it would be putting too much responsibility on them - they'll be home anyway - and they should already know what to do to take care of the younger siblings from jsut living there. They'll be ok.