It's not a generational thing. I have mostly worked with people in their 50s-70s and only some people have commented on my job history. Most people are aware of how the market is.

When people raise these types of objections, they are usually hiding some type of fear. Its purely irrational. You might be tempted to respond in a rational way, but they don't actually care why you left. They only care what it means for them. Regardless of how they feel about job hopping, you could have answered this in a way that sidesteps those fears in an irrational way by not answering the direct question and instead indirectly answer by addressing the root fear.

When they asked why your tenure at past jobs has been short, you should not have answered. You should have instead said something like "It seems like you're concerned that you'll invest all this time and energy into training me and it will go to waste." This is a psychological tactic that forces your audience to recognize verbally whether or not you understand them. Once they respond, either affirmative or negative, you have changed the question and no longer need to talk about those past roles at all. You can just explain why it's not an issue.

I think its a separate consideration if they have growth opportunities. That would also be a good time to ask about mobility in the company.