Weather: the only thing to do is to phone up before hand and ask if they're flying today. Try and book late morning/afternoon slots if you live far away? Also, find out what the schools minima is for PPL instruction, then you can check the weather yourself and start making decisions for yourself. You'll find that in the winter you ought to be able to get a fair bit in, I found that over the summers I did my licence there was a fair bit of cloud and crap vis which prevented flying.
HP Exam: If it's in the book then it's useful information to learn. Get a confuser/simplifier so that you understand how the questions are structured. This will help put things into perspective.
Instructor: It's probably more important to get an instructor you get along with rather than choosing one because of their age. Remember you're going to be shut in a small space with them for a long while! Consider more than one instructor too?