I don't talk about it unless pushed.
'When are you having children?'
'I used to feel that way too, until I grew up.'
'Ah, you'll change your mind.'
'People like you should have kids!'
'Who's going to look after you in your old age?'
'You just haven't met the right man yet.'
I find that many people are so desperate to validate their own life choices (this isn't limited to having children, could be anything) that they cannot comprehend that other people have made different life choices and are happy.
Some of my friends have children. They know I've never wanted children. I know they've always wanted children. We're friends because we respect each other's life choices and don't constantly try to force our opinions on each other.
I was incredibly happy for them when they got pregnant because I knew it was what they wanted. They are happy for me that I remain child-free because they know it's what I want.
It's really not difficult to accept different people have different wants and needs.