How do I determine the settings to use to age my characters appropriately?
"Younger/Older" is somewhat arbitrary - whereas I tend to know (at least vaguely) the age of my characters. How would I create, say, a 35-year-old man in my face room?

And should I do this before or after visiting the hair room?

You will need to determine  which features require adjusting to give you the appropriate age you want.  There is no formula, just spin dials.  Which figure are you using?

Unfortunately, age is an arbitrary thing. I've seen women who are eightty, but look and act like their sixty, and vice versa. How old would you say P7 Kate is? Or Roxie? Or Alyson? The thing is, they fit into a range. Turn the dial until you thing they look like the age you're aiming for.

Okay, so no helpful tools or pages to use...  Yes, I know that different people age differently, but it would be useful to know where on the spectrum (a typical) someone might be in their 30s or 40s or 50s or whatever...

Thanks for your help.

Do a Google image search on "aging"

I found some excellent examples there.