Regarding inexperienced players, I think it is just that it's better odds that someone with less experience is also possibly younger. By no means a hard and fast rule, but more likely. Plus, when giving advice, it helps to know your audience so you can speak to them on whatever level is most likely to help them understand your points. I agree it is better to ask how long someone has been playing than how old they are.
As to bad players, age knows no limit, but bad players seem to be in the extreme minority because RPGing tends to attract a somewhat sharper crowd, IMO. And people who are bad at something, when they aren't forced to keep at it, tend to drift away from it. Hobbies are meant to be fun and it's rarely fun to keep at something that you aren't doing well.
Anyway, those are my thoughts/theories on those matters and I'm sure they aren't original.
A Professor of
D&D-ology?!?!