I think it really depends on the young person.
Some really want to learn to cast a fly rod and some it is only the parent of Grandfather wanting. When the young person really wants to lean, I feel 5 is not too young. My two grandsons asked me to teach them tofly fish.
Not having a cheap rod, I places one of my good rods in the hands of a 5 & 7 year old and started teaching both of them to cast. Attention span was not as good with one but the other was outstanding. (my reason for saying it depends on the young person) I would say to not push. Allow the young person to progress from casting on grass to water at a speed that suits them best. Be prepared to untangle knots and replace tippet that is not fishable.
The last visit the boys made to go fishin with me, the now 12 year old landed a 16 " bow before we even got a fly tied on the 10 year olds tippet. He actually had the fish to hand and already unhooked and was releasing it before Ben & I got to him. We were all proud. His choice of fly was a Chile Pepper, size 16.
All 3 of us caught several fish that outing and I am looking foreword to them visiting to go "real fishing" their term for fly fishing, late this month.

Denny