Answered Jan 4, 2019
It’s two time-based issues.
First, the biological clock.
No, they aren’t worried that you might miss the window of opportunity to have the fun experience of parenting. Rather, they are worried that you might get away with shirking an alleged obligation to have kids.
They need you to eventually have kids in order to validate their own lives. To confirm that, literally everyone has kids, and nobody has a choice. Because many of them actually refuse to believe that they themselves had any choices. Including making poor choices that they regret. Via contraceptive negligence, with the unplanned child they believe just “happened”. Or by buying into the societal script of deliberately having kids, because it’s just what every real adult supposedly does.
If you were say, 25, and didn’t have kids, they would still hold out hope of it “happening” eventually. If you said at that age that you never want kids, they will be condescending, and treat you like you just aren’t old/mature enough to grasp the compulsory breeding that you “will” engage in when you become a “real” grownup, like maybe age 30.
When you get past 30, they start feeling entitled to an explanation or apology for what is “wrong” with you. Or a reassurance you are now accepting your fate, and practically ready to have unprotected sex with random street bums, just to produce the required 2-point-5 kids.
The second issue is more general. It’s called, “settling down”. It’s largely based on the idea that, everyone’s life trajectory includes getting married, having kids, being trapped, and then never doing anything new, or making any choices ever again. It also includes things like remaining within a very limited geographic area, being stuck in a job you hate, and never pursuing any more formal education.
“Settling down” means that your life story is now over. And they want to see this happen to you as soon as possible. They will try to convince you that you have fallen behind schedule, and are immature, unrealistic, irresponsible, unstable, etc. if you don’t have the handcuffs on (yet).
The people who “settled down” long before 38 may feel personally insulted, and entitled to socially retaliate.