If you are in your 20's or 30's in doubt of having kids or not, and always finding a reason to not have them, you will probably regret skipoing. But if you, like me, know for a fact that you don't want kids and you never felt any kind of desire to have them, you will probably not regret it.
I'm the only one in my job over 40 that doesn't have kids. And both the quote from the article "By the same token, why does nobody ever say that having kids was a mistake?" or yours "Everyone I know with kids in their 40s and 50s is content." are false. If you poke hard enough, moms fall apart and recognize that sometimes they find themselves wishing to get rid of their kids and fantasizing about a childless life. But they don't like to talk about it because that would make them bad mothers. I reassure them that it's a normal feeling if you are going through hard times with them.
But they are not specially happy with having kids (and I'm sure there are precious times with them), nor I'm sad or regretful for not having them.