Hmm, Well if you want the opinion of this lowly brown belt (Adult student), I think the best student is a teen... They are old enough that they have discipline when needed, but they have nearly the same enthusiasm, imagination, and fearlessness, as a kid. on the negative side they learn fast and are always asking for more regardless of how sound the techniques are, and you can run into the issue of puberty between your guys and girls. All, in all I think every student as an individual has pros and cons, As an instructor you need spot them and correct, workaround, or embrace the differences.