Yes, 44, mainly through British books and television programmes.
Yes, and 47. I just asked my 20 year old daughter and she said, No.
24, and I had no frickin’ clue, until I asked Wiki.
28, never heard it.
I thought I was relatively well versed in fables but I guess not.
And I Wiki’d it too.
27 and yes.
Yes - 45
I can’t tell you why I know it - one of those things my brain has held on to for decades with no memory of how it got in there.
[QUOTE=twickster]
Do you know what this expression means? How old are you?
(I just used it while talking to my sidekick, and it turns out that none of the three people in the office – two are about 30 and one at about 35 – had ever heard it before.)
(FTR, none of these three people has ever seen The Maltese Falcon, either – I don’t know if there’s a connection there or not.)
[/QUOTE]
Yes, 26, but I had a book of Aesop’s Fables when I was a kid.
Yes. 32.
Yes. I’m 35. But my parents were in their early forties when I was born.
Yes. I’m 43.
I’m 42 – and all this is just a secret plot to get our ages, isn’t it? – and to me a dog in the manger is the same thing as a wet blanket or a stick in the mud.
[QUOTE=Sal Ammoniac]
I’m 42 – and all this is just a secret plot to get our ages, isn’t it? – and to me a dog in the manger is the same thing as a wet blanket or a stick in the mud.
[/QUOTE]
So you’re not actually familiar with the fable whence it came? 
Know it, about the same age as those in the OP’s office.
Of course, I recently had a dog who was like that in personality, so the characterization is now burned in my mind.
I’ve heard it before, but never knew what it meant (until 3 minutes ago). I’m 57, but I don’t believe I ever heard it until 10 or 15 years ago, probably trotted out by some political pundit.
27, and I had never heard it before. I do know the meaning now though.
34, and I know the fable.
24 and yes. My primary school teachers were big on Aesop.
56 and yes. Not only from Aesop’s Fable books, but from Aesop and Son on Rocky and Bullwinkle. 
(I’m actually not sure if they ever did that one.)
52, probably never used it, but have known it since I was a child. May never have heard it used, either, but I’ve read it several places, the first time being, as everyone else has said, in Aesop’s Fables.
Now I’ve got to check to be sure the kids all know.
- Knew it from early on. I think I heard it directly from one of Aesop’s pupils.