babyboomersrock ... but you're not going to find yourself sitting in a B and J booth, muttering passive aggressive remarks to a young mother, are you? Precisely because you have a good deal of social capital? Because you wouldn't regard the young women as a liminal figure, to whom you would direct your anger?
My guess is that the mutterers muttered because they felt insecure, and felt anger, however much of a misperception that might have been.
Some men attack some women because they feel anger towards what they think those women represent for them. I remember, years ago, a complete strange trying to push me in front of a car, on a busy street, in broad daylight. He didn't know me. I just represented something for him. I threatened him in some way, and threatened him enough for him to fill he had to kill me.
I would never say that one man represented all men. Or that his response was an automatic one. Indeed, I was actually rescued by a group of men coming to help me - proof that not all men/people react to me the way that one hostile, man did. However, that one man did misperceive something incredibly threatening and anger-inducing about me that made him want to kill/maim me.
Anger, fear, and power are interesting, and have interesting, and different, effects on different people. Just because you wouldn't react like that to a young mother doesn't mean all people, even all older people, wouldn't.
I don't think you can dismiss the OP's experience with mockery or by telling her she is being ageist.
Ageism, as in anti-old people hate-speech and action, definitely exists, but so does hostility towards younger people, especially mothers and children.