9 years. At this age the older one is a REAL help! you can actually trust the older one to care for your newborn while you have a shower... they can often walk to the corner shop to buy some milk... they can often walk to school by themselves meaning you don't have to wake baby to do the school run. it is so easy (LOL, as if!) that you can then go on to have a third baby and by then the oldest one can be a babysitter! not many negatives to be honest. My eldest is a DD... my two younger ones are sons. DD and I are still close... she was old enough not to feel too jealous etc...we still go out shopping or to the movies together.