It's called "exploration vs exploitation". Unless you meant the idea that older people need to explore less because they're already done it. I don't think there's a name for that specific idea.

I originally searched for "exploration vs exploitation" but that lead to the "multi-armed bandit problem" on wikipedia, so I went with that on my post above.

I do that "exploration vs exploitation" is a better name for it though.

Yes, the general name is "ageism". This has nothing to do with young versus old. Younger people are funnelled to a small number of larger sites just as much as older people.