I read that it was because of production problems that meant customers couldn't find items they had seen advertised when they actually went in. Now I know that has been an issue, but I also thought typical M&S, ignore the actual issue which is that your clothes are awful!
I want nice fabrics, proper cotton, linen, viscose not polyester, and wool not acrylic. Then make these into useable garments (like a straightforward v-neck jumper, work trousers or shirt dress) that are classic and unmessed with, but up-to-date in cut.
STOP chucking stupid frills and fake leather bits all over everything. It adds cost and is horrible.
You can charge more, I'd rather pay £50 for a quality jumper that will last, than £25 for a synthetic pile-o-shite.
Reduce the number of items and concentrate on stocking them all in all sizes.
Get rid of all the daft "collections". I don't want to wander around a massive hangar looking for my trousers in 8 different locations. I want to go to trousers and see what you have.
I don't think it's an age thing - plenty of shops have clothes that appeal to multiple ages. Topshop, Primark, Zara. It's a styleless, messy, quality nosedive thing.