I know several women in their late 30s/early 40s who do not have children. I know one is not childfree by choice (close friend) but have no idea about the others apart from two who have actively chosen to share that they have never wanted kids. I do not spend any time at all wondering why the others do not have children. I assume they don't want them or, as is sadly the case of my friend, are unable to but it isn't something I dwell on or speculate about.
I've read multiple columnists in newspapers and articles in magazines in which child free women declare that the public at large, and mothers in particular, are 'obsessesed' with women who do not have children and it's exhausting having to constantly explain why you do not have any etc. They are fed up with the general assumption that they are selfish, career-mad, horrible child haters and this sentiment mostly comes from women who have kids. I have DC as do most of my friends. In my 10 years as a parent I have never had a conversation with any of them about 'selfish' childless women. Most of us completely understand why someone would prefer not to have kids!
I might be alone in thinking this but I honestly don't think most people really care about a woman's childbearing status. Obviously there are many ridiculous individuals who think it's ok to question a woman's choice and trott out BS like "you've never known true love/what tiredness really is until...." but I honestly think they are in the minority and the majority of mothers do not really care. Or am I being naive? My friends who are childfree by choice tell me they have occasionally been asked if they have kids but with rare exceptions have not been asked why not or any other follow up questions.