In my experience "Smile darling" and other such interjections from random men in the street is subtext for "Look at me I need attention"... It's usually a clumsy half baked chat up line (I have never experienced it from a woman), I'd say if it's from a stranger it's fine to ignore it - other reactions I've given have included a fake cheesy grin or a glare depending on my mood. It's darn right rude to talk to a stranger like that so it's a justified response IMO.

In my experience these things didn't happen to me once I got older, adding to my theory they are just half baked chat up lines.