Sometimes life gets in the way of our plans. There is a 9.5 year gap between my two DDs, and it matters not a jot. They are smitten with each other, they enjoy each other's company, and as for socialising, they are both very good at forming bonds with a beautifully diverse range of people. I wouldn't advocate leaving it quite as long (for people who do have a choice) because there are logistic difficulties (eg, I've been slave to the double-drop-off for years & handmedowns aren't as easy) but on the other hand both of my girls have had space to grow into their individuality.