I think 40 is a little young, but maybe 50 or so. I was watching a Younique presentation lastnight. The presenter said there are no rules in makeup, women should wear what they like and feel comfortable in. IMO, that's a selling strategy because she has to market to a broad audience. The presenter is in her early to mid 50's and used shimmers in her demo. She didn't look bad per se and can still get away with it.
I personally believe that some of it has to do with a woman's overall appearance, if she looks her age or not, and the condition of her eyelids. Someone could be on the younger end, have bad lids, and shimmery eyeshadow isn't going to work well for her. I'm 60 and tried playing around with a couple of palettes I hadn't used in awhile. Whew, that was bad. I've never used metallics, but even some shimmers are way overboard. Those palettes got pitched. For me, I prefer matte or maybe a very light shimmer or satin, but that's as far as I go. My eyelids are in pretty good condition. I use skincare products on them and exfoliate them, to keep them smooth. I also use an eyeshadow primer.
Colorwise, I usually stick to lighter shades in brown/taupe/champagne or grey and light charcoal as my go-tos, but I'm not opposed to using some colors. The one big change I've made is that I don't use traditional eyeliner anymore. I will smudge some eyeshadow on the lash line for the smokey eye effect, and sometimes I go underneath the top lash and tightline with a flat angled brush. For me, I find that it makes the lashes look fuller w/o taking up good real estate on my eyelid, if that makes sense.