I think the term began as referring to mid-life. Well then why not call it mid-life singles? No. Probably not a good idea. Sounds too close to 'mid-life crisis' and some of those result in once married people becoming single.
I have some very dear friends who have achieved this status in life and wonder if there was a more appropriate designation.
I like sports, so maybe there should be a major league singles and a development league singles. Think about that! Pretty sure I went through the D-league for a while there at some point.
Of course, I don’t have an answer that won’t offend someone, but at the end of the day why should there be any additional distinction beyond single/married? They certainly don’t ask whether you’re single or mid-single when you file your taxes. There isn't an additional 3% credit to claim.
Call it what it is. If it looks like a duck and walks like a duck?