If I saw a woman over 40 without make up my thoughts on her will be coloured by the occasion (gym, hospital, wedding), the context in which I see her (recently bereaved, just woke up, recovering from surgery, walking the dog), how the rest of her appearance looks in terms of style and grooming. Her accent and voice as well, does she sound like a posh woman,a rough accent, an essex accent?
If she looks good I would wonder if she has no make up make up on, of if she works in skin and beauty so has access to all the best tech or if she is a rich woman having a day off (sports, tired, feeling down, letting skin 'breathe').
If she has skin issues that would look better with make up like pigmentation, dark circles, sallow skin (basically how I look without make up) then I would wonder if she is ill, depressed or doesn't know how to apply make up (because that's my situation) because I know she would look better with some and it doesn't even take much time or products. The only times I haven't bothered were when I was too sad or ill or both to bother with make up.
If she looks average, depending on her mannerisms, voice tone and clothes I would wonder if she is a butch lesbian or religious or super feminist.