I have always thought of adults over 30 without children as "Adult Children"

Why?

I realized when my first child was born exactly how selfish I (and narcissistic) that I had been as a bachelor. When your life becomes more about others (like your children, your wife, your Grandchildren) then it changes a person a lot. While friends of mine were taking many trips to places like Thailand, Japan, India and Nepal and sometimes spending 6 months or more places like this in the 1980s I was busy putting my kids through school and getting them into college. I was only able to make one 4 month trip to Japan, India, Nepal and Thailand myself in 1985 and 1986.

So, I tend to think of childless adults as literally "Adult Children" and I treat them entirely differently than I do people with children or grandchildren because they are more like my children than they are like parents or grandparents who I find to be much more adultlike usually in their behavior in life.

However, even I am realistic to realize that being a parent isn't for everyone. However, when I researched my father's family line back to 1580 in Switzerland I found that every one of my direct ancestors was married and had 5 to 10 children up through my father's family of 5 children who were born between 1912 and 1925. 

So, what I'm getting at is that my whole genetic line on my father's side has been having 5 to 10 kids, being married, owning their own businesses etc.  for well over 500 years or more in Switzerland and here in the U.S. as well. So, this is part of my genetic heritage.

The other thing to think about is that people who don't have children their genetics don't move forward into the future either. But, I don't criticize people for not having children because it isn't for everyone. However, I see childless  people more like adult children who are more like teenagers or 20 somethings than anything else too.

It's mostly a choice of what you want to do with your life and what is important to you in the end.

But, I definitely treat childless people differently than people with children or grandchildren because they see things so differently to begin with.