I am 47 and own two slips. I haven't worn them in years because:
1. I can hardly even purchase a dress or skirt anymore that isn't halfway up my thigh, so hardly wear them anymore.
2. When I do purchase them, they are usually heavy enough that I no longer need a slip.
I have two, longish white linen skirts. Even though they are lined, the lining is sort of skimpy so I put my long white slip under them.
I have a long, "woolish" black skirt that can be itchy, even with it's own lining so I put a slip under that.
Other than that, I no longer need one.
I do wear pantyhose whever I wear a suit or something "conservative" or formal business wear. Today I have a flouncy pink/beige skirt on with no hose and sandals. No slip either as it is nicely lined.
For the person who said that women now wear Spanx--well, I find Spanx horrible uncomfortable and binding. Pantyhose are way more comfy to me.