It's not an age thing or a regional thing. I encounter it everywhere and it annoys me. I was on a train at the weekend packed with people returning from a football match and when I tried to navigate past standing people I was met with "sorry darling" from the men.
Workmen have pitched up outside the house (they're doing work a few doors down) and a guy in his early 20s called me "love".
Maybe they're being kind or think they are, but they aren't, and even if they don't know it and it's a reflex action, they are being patronising and putting us in our boxes. That we are their darlings no matter how old we are. I'm early 40s. I've had it all my life, and my grandmother gets it too. The age of the woman on the receiving end or the man saying it is immaterial too. It's as if guys learn this at puberty. I've had it said to me by kids on the bus in the morning.
I usually respond by saying something like "that's ok (insert silly term of endearment here)":
Sweetie
Babyface
Cutie patootie
Honeybunch
Kitten
The more stupid, the better. Nobody has ever challenged me or looked gormless at me but if they did I would tell them why.
I'm thinking of adding in son, or kiddo, if young lads call me darling or sweetheart.