I guess every child has different capabilities, attitudes and attention spans.
My little spawn of Satan seems to work the wee bait rod ok (3 ft tip section of fly rod with a half wells handle and up locking fly reel seat coupled with a tiny fixed spool reel. Backing off the drag a good bit lets the fish please itself without snapping everything or him getting a soaking.
The last trip out (Lochter) I took the 2wt TXL out and threw on a dry, referring to the old clock face analogy he loves to use the 9 to 3 arc which I Imagine is the industry accepted one in kiddytown. Any attempt to assist was greeted with the usual growl and request to cast for him, which wasn't an issue as the aim was to hopefully introduce the excitement of a trout rising to a fly and the feel of a fish on an outfit which isn't hindered by the resistance of a bubble float.
Once he got a bit of a feel for keeping a bend in the rod, giving the fish line, etc. I think he enjoyed the experience.
As for him actually taking more than a fleeting interest in the casting side of things..........I think he is too young and will be quite happy to have me throw out a dry for him for a while yet, fine by me.
Greg, absolutely, 5 years old is not too young - time at the water is never wasted and casting a dry or 'Klink & buzzer' 10 feet on a lightweight outfit is both easily achievable and more than enough to cover a heap of fish at a fishery.
Best of luck
Iain