Well if you never told people that you already know these things, then you can’t blame them here.

I don’t think assertiveness needs to be rude, you can tell them politely

In a professional world age does not matter, well it doesn’t matter in many other instances either. I’ve met plenty of 20 year olds way smarter than some 65 year olds. Age doesn’t always equate knowledge or wisdom. Many young people know more than me on many subjects and I have no problem them teaching me. Age doesn’t matter here

Of course there are instances when you can just let them talk. Like at a dinner party I see how you don’t want to start telling them they are wrong.