Question: Some people do not move out of the way when walking even if it would be considered rude or socially unacceptable, like when the other person is much older, pregnant, or has small children. They expect others to move for them, but they aren't breaking any laws or rules, rather not following social norms.

This may seem very trivial but are there are cases where something like this escalated and police were involved?

Who is responsible if a child, teenager or young adult does not get out of the way for an elderly person and the elderly person shoves them out of the way, knocks them on the head or commits some other act of physical aggression, which happens sometimes in such cases?

Where do you draw the line - if the older person only gently taps them so they move is it still their fault because any unwanted touching is assault?

I have also seen elderly people become physically violent towards younger people who don't give up their seat on a bus or train, although in that case the younger person is not following rules.

I understand the older person is committing assault but most people even in first-world countries like Canada either support the older person or don't interfere and the law would have to take into account the common public opinion.

Answer: 
There are some instances where you might be allowed to do something only because it follows prevailing social norms. In the U.S., the Supreme Court has held that [“A license may be implied from the habits of the country.”](https://www.law.cornell.edu/supremecourt/text/260/127)

For example, the U.S. Supreme Court in [*Florida v. Jardines*](https://www.law.cornell.edu/supremecourt/text/11-564) determined that the police (like anyone else) are allowed to walk up to someone’s door and ring the doorbell or knock on the door, even though violating commonly-understood social norms could constitute an illegal search of someone’s private property, or even trespassing.

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> This implicit license typically permits the visitor to approach the home by the front path, knock promptly, wait briefly to be received, and then (absent invitation to linger longer) leave. Complying with the terms of that traditional invitation does not require fine-grained legal knowledge; it is generally managed without incident by the Nation’s Girl Scouts and trick-or-treaters.
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(Yes, you guessed it: that sarcastic closing line is Antonin Scalia.)
Sometimes social norms matter.

This comes up not primarily in the area of criminal law and intentional torts like assault and battery, but in the area of negligence.

Suppose somebody doesn't get out of the way of an obviously fragile person and that causes the person to fall. Is the failure to get out of the way actionable?

Maybe.

Everyone has a universal legal duty to take reasonable care to take necessary steps prevent others from suffering physical injury or property damage. What a reasonable person would do under the circumstances is determined by the trier of fact (the jury in a jury trial, or a judge, if there is a bench trial) on a case by case basis. This can be based, in part, on social norms.