This plus …

Time as you perceive it, is related to new memories you make.

When you’re young, everything is a new experience which in turn becomes a new memory.

When your 100, to use your example, you’ve done everything there is to do. So no new memories & days blur together.

So, if I suddenly have amnesia and don't remember the past 99 years, will my time slow down?

It is just a thought experiment...

That's the explanation I came up with for myself, too. As humans, we rate most things not in absolute but in relative terms to what we are used to (see studies about happiness or how rich people don't realise how wealthy they are when they don't leave their bubble). Why should we perceive time differently than in comparison to our timescale?

The older you get the less you learn and the less new things you learn. Days are becoming repetitive. Looking back at last year contains the same information as 1 week when you were young.

That the mind compresses oft-repeated experiences makes more sense than this math-as-psychology nonsense.

Notice how when you have to wait for an hour, you’re bored, time seems to pass slowly, yet for those last ten minutes time passes more quickly because there’s only 1/6 hours le— yeah exactly, no, that doesn’t happen. The whole hour passes slowly because that whole fraction theory is bunk.