Late starters! My 2 year old absolutely adores flying. He's been doing it since 6 months, though regularly only since he was around 2 (nearly 3 now). He now has his own headset (as advertised above); he likes to sing to himself on the intercom so it's a good idea to have one of those fancy intercoms that can isolate the pilot and/or front seat occupants (or unplug him strategically)...
I wouldn't take him on my own, though, for quite a bit yet - either his mum or his godfather (also a pilot) always comes with us.
Different, I know, from taking an older child who hasn't been flying before: he's too young to have anxieties about it and should, with luck, grow up just thinking flying is normal. But he talks about it for days before and after every trip!
Tim
(PS if anyone's got somewhere to host it I have a very cute photo of him asleep in the back of a Robin DR400...)