I don't have children and I was always expected (not asked) but expected to work holidays because 'people with families' should come first. Extremely frustrating because like others have said, I do have a family, just not my own children.
I can not even begin to tell you the number of times I'm having a conversation with someone, most times a complete stranger (new neighbour, someone at a shop, hairdresser, etc) and somehow the conversation turns 'What about your kids?' or 'Where do your children go to school?' or 'How do you manage all of that with children?' (because it's assumed a married women has children, this didn't happen to me until after I got married, prior to getting married it was presuming I was married)
I always, reluctantly, reply 'I don't have children' (because I know what's coming next) ...it's either a look of shock, or of pity. Then they say 'Oh, I'm so sorry hun, why not? Are you having problems or putting your career first? You know you could always adopt!'
I've just met you, the last thing I want to do is discuss my personal situation with you. It's really none of your business and feels such an invasive question. (prior to getting married it was 'why are you not married yet?')
Then as I try to leave I've had people (both men and women) say, 'well, you'd better get a move on because you'll regret not having them when you get older!'

My advice to people asking what to say/not say, firstly don't just assume all married woman have children (or that all women of a certain age are married). If you happen to ask someone and they say they don't have children, don't ask them 'why not?'. If they wanted to tell you, they would. Don't then end the conversation by saying something patronizing like 'You'll regret it if you don't do it!'. If they wanted your advice, they would probably ask for it.