my eldest daughter is 13 and has been babysitting her younger sisters for some time, admittedly only an hour at a time usually, and ive checked legalities on this and i am quite within my rights to allow her too.

having said that she is very mature and her sisters are also very well behaved, they know what to do in an emergency, they have all been taught to use the phone correctly, they all know my work and mobile number off by heart and we are fortunate enough to have wonderful neighbours whom they can run to at the drop of a hat.

it comes down to educating them on responsibility, setting rules in place which are easy for them to follow when your not there, my girls have chores to do which keep them occupied for the hour before i get home from work, they get pocket money accordingly and my eldest gets an allowance just for babysitting, this in itself gives her a motivation to be responsible.

i also had to do this is as a 13 year old and i see no harm in it as long as your teen is mature enough and they dont feel you are taking advantage of them, they are not parents so guidelines do need to be set.

Sorry my computer is playing up.  I was babysitting at age 14.  Depending on how immature you think your teen is, you should give him/her a go.  That is the only way to find out.  Maybe you should find out how your teens act around younger children.  See how they interact before you put them in a baby sitting job.  This can give so much insight to how your teens interact with children they will potentially baby sitting.