Hi,
I recall learning to fly fish when I was around 9 or 10. I would think if your grandson shows an interest in trying to fly fish, then let him give it a go. Probably best to see if he can do anything that even resembles a cast first. If he can hold the rod well enough, and if he can cast the line out a ways (does not have to be far), then take him to a small river or creek and let him drift some wet flies around. Wet flies are the easiest flies to present since a bit of movement in the line is fine, and drag is not a bad thing.

Lake fishing would be harder if the rod is too big for him to cast, and he might get turned off. Of course, you could try casting out a set of midge pupae, or small nymphs, and then give him the rod to hold since they can be fished static along the edges?

- Jeff