Pilot Mike has given a well though out description of what I have also experienced in training. In particular, younger students seem to learn faster. Considering the accident of this discussion, a young and apparently eager student would be just the type of student who would inspire me to train them, that makes it extra sad.
I agree very much with Pilot Mike's assessment of older "type A" personality pilots. My instructing has been limited to advanced training and seaplane endorsements, both of which attract well off type A personalities (who can afford the planes). After a few scares, I am very much on guard for pilots who over estimate their abilities, or don't respond well to guidance. I now give fairly stern briefings as to what will be acceptable, and not. Maneuvering, and the risk of stall/spin figure centrally in my type training, as typically floatplanes and amphibians have higher wing loading, and more drag, so recognition of the impending conditions is extra important.
Young, eager students are my favourite, but as I do not do ab initio training, I don't get to fly with them very often (hence my question about solo times).