I think of long hair as “bra strap” length or longer.
I’m 20 and cut off my long hair when I was 18. Boy did I regret that. I cut it to my shoulders and it was down to my belly button! I looked like a 14 year old with short hair. So I had my 1 year of short hair and now it’s to the “bra strap” length and I’m loving it once again. I do plan on having long layers - since that seems to be the best look for me.
Long hair looks good on most women as long as they take good care of it. I will add that I don’t think it looks as good if it’s cut straight across. I think that looks very plain and aged. As others have mentioned, ratty & splt ends always look bad. Trim it up and keep it pretty!
For older women I’d suggest shorter long hair with some shape. A woman I used to work with had about arm-pit length hair brunette hair, with layers and long whispy bangs. She looked younger than her age (early 50’s) and it also made her face look a bit thinner (she wasn’t fat but she did have a few extra pounds).
And of course… If in doubt of your decision, cut off less than you planned!
Don’t go for the old lady mop!!!