The first time we could afford a WDW vacation, our "kids" were 17 and 19. It was a huge success, and we have returned many times since then. I was fortunate enough to be there with my first grandson when he was 8 months old and His parents and I have the most magical memories ever of watching his absolute delight at the sights and sounds. Watching him wrap his chubby little arms around Goofy's nose was priceless! Obviously, that trip wasn't planned for his benefit. His parents had to use a travel credit, and were not ready to be away from him, so I tagged along and kept him close enough for comfort while they could still have some time to themselves.
A few years later, we made a 4- generation trip, with 3 grandchildren, ages 2 - 3. WOW! Incredible magic. And the pictures and videos of that trip fuelled their excitement for a trip 2 years later, and so on.
The absolutely perfect age to go is when you have your health, the desire, and the financial means to do so. Never, ever take any one of those elements for granted. This may be your only trip, or you could be like us and find yourself becoming regular visitors. Never in my wildest dreams did I expect to make a second trip, let lone at least every other year for 24 years! Orlando as a destination is fantastic for fun of all kinds. DH has tapered off his park time considerably but is still happy to come along.