@Inextremis
DH is 58, and goes out with the 'boys' to the pub. It's colloquial, not disrespectful - if I go for a drink with the 'girls' it's highly likely that none of us are under 50, and very unlikely that any of us would be offended. Each to their own.
It is about context as other posters say
I may refer to a group of women I hang out with socially and know well as “girls” even in our 50s and 60’s. It indicates a sense of playful, unserious gathering and socialising. We’re not going to behave as “adults” , we’re just hanging out
But, I will, and have called out anyone calling me or other women “girls” at work. It is a professional setting and it labels women as unserious, less authorative in the speakers mind. More easily dismissed . That’s at best. At worst it is completely patronising and deliberately so.
I do also take strong objection to people I don’t know, especially men, calling me or even groups of women “girls” even in more social situations. Much the same as I do if they try to call me “young lady” ( I’m in my late 50s. Again at best it is ingratiating (e.g. from a waiter who thinks I’ll be flattered and leave him a tip), and at worst it is sexualised or overfamiliar attempt to muscle into a group of women hanging out.
So context.