3 1/2 years has worked well for our two, One is protective of his brother and on the whole kind to him, Two adores One and thinks he's absolutely hilarious. But since Two is going to be a late walker, it will be a while before they can really play together, and meanwhile One is moving on. Personally, I was nowhere near ready with a very young toddler to go through pregnancy again, so the "traditional" two-year gap wasn't an option for us.