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: 
Rudeness is not a crime
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Thankfully, or I’d be writing this from jail.

The threat or actually of intentional and unwanted physical contact *is* a crime. Historically these were seperate common law crimes (and torts) of [assault](https://en.wikipedia.org/wiki/Assault) and [battery](https://en.wikipedia.org/wiki/Battery_(crime)) respectively. However, in most jurisdictions these have been codified and merged and redefined so there is common assault (which merges historic assault and battery and is what you are asking about), indecent assault, sexual assault etc.

In general, there must be an intentionality to the contact - accidentally colliding with someone is not assault (although it can be the tort of negligence). Similarly the contact must be unwanted - participating in a game of rugby means you want to be tackled (in a legal sense, in a sports sense you don’t want to be tackled, you want to score). And finally it must intend harm - holding your hands up to prevent a collision does not intend harm even if harm may result, punching someone does.

In the cases you describe, the aggressor is the “elderly person” and they have committed a crime.

I am interested in the suggestion that public opinion in first-world countries like Canada is that being old and annoyed justifies you beating someone. That is not my understanding of public opinion in Australia- old people have to follow the rule of law here just like everybody else.
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.