How old to babysit siblings?

How old would you be comfortable with your oldest child babysitting the youngest child? Or would you base it more on maturity? Thoughts?

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      Maturity.. showing responsibility.. trust.. I think I would let my son watch his sister.. even though they're 2 years apart when he's older for sure.. (7 and 9 now).. he's super responsible and trustworthy and almost a worrier or scaredy cat to my liking so I know he won't fall off my rule path..

      My daughter.. is just different.. a little flightier.. she's the type that would make cookies when she's not supposed to.. :)
      About Melissa
      Birth: December 31
      On since: Mar 3, 2014
      I am a single mom of two fantastic kiddos that I love to pieces. Currently in school working towards my teaching degree. You can find me most days on when I am not here chit chatting! :)