To WDW, they were 15 months and almost 7 years old. To DLR, they were 7 weeks and 9 months. We have great memories of all of it, even the 7 week old where we held her up around various places and took "selfies."
But my very favorite ages for each kid were just before they turned 3. They were tall enough to ride almost everything, they loved the magic and full of wonder, and they were free!