I know the phrase but don’t remember ever once hearing it used in an actual conversation. I’ve seen it used in movies or TV and have read it as an allusion in stories or news articles, but never once in Real Life.
Oh, and I’m 37.
45 and I know it but only because it’s used regularly in the books I read - regency novels. I’ve never heard it outside of that context.
58, Yes, I’ve known it since childhood. I remember being confused, because there wasn’t any dog in the Baby Jesus stories. Wrong manger. Then, in December, we taunted the Sunday school teacher, singing A Dog In A Manger.
A dog in a manger, no fleas in his bed.
A little brown doggy lay down his sweet head.
The sheep in the stable could get no fine hay,
The dog said, “If you come near me, you’ll pay.”
Actually, no. It’s just an expression to me.
Wow. How about “crying wolf”? The hare and the tortoise? The ant and the grasshopper?
Please! La Fontaine is the only fabulist I can stomach. I have no use for that poseur Aesop.
29, I know it, I still own the book with Aesop’s fables I’d read it in originally, but have never seen The Maltese Falcon.
I was never taught Aesop’s fables in school.
Skipping the other answers, I’m 39 and I think I may have heard it but I couldn’t tell you what it meant. I probably heard it on the Dope.
Yes, 36. But, I’m foreign.
Know it. 60.
37
Dog in the manger? Vaguely familiar, but wiki informs me I was wrong. Don’t remember ever reading/hearing the fable.
Sour grapes I knew, but didn’t remember the actual fable either.
Crying wolf, the hare and the tortoise and the ant and grasshopper I knew, although the DitM experience made me double check them.
I went to a religious school for elementary, all our fables were parables, and I don’t remember reading Aesop at home or in any high school lit classes.
I do remember all the ones that were made into Disney cartoons, though! 
No, 19, which is weird because I know crying wolf, sour grapes, tortoise and the hare, ant and the grasshopper (and the Futurama version), lion and the mouse, wolf in sheep’s clothing, town mouse and country mouse, and probably a few others.
We learned some fables at my elementary school, which was Lutheran; others were in books my parents or grandma read to me.
74 and I don’t know what it means, what the Maltese Falcon or Aesop had to do with the term, and I’d like to say I don’t give a rat’s ass, but I do.
So please tell me and all the other ignorant dopers as well.
Which is truly the best version. Who wouldn’t want a cool racecar? I’m totally going to grow up to be like the octopus.
- Yes. Had a book of Aesop’s fables (and just about other every myths/fables/folktales book in existence) as a kid.
JRB
I’m 48 and remember my mother’s use of the phrase when I was a kid, which I recognized when we learned Aesop’s fables later in school. Never seen ‘The Maltese Falcon’. I wonder if there’s a regional factor in whether one’s heard it? (FWIW, I was born and raised in West Virginia).
Read here. 
I had to laugh at that, it was quite good!
50 here. I’ve heard the dog and the manger, the fox and the grapes, and quite a few other fables. I also remember Aesop & Son from the Rocky and Bullwinkle Show.
54-know it well.
54 know it and its origin and use it. Have seen The Maltese Falcon many times, have read the book 3 times and own a replica of the falcon from The Mysterious Bookshop in New York.