Growth in this context is that there are a larger volume of videos each year. So each year a single video is exponentially a smaller and smaller percentage of the total.

Yeah and the math doesn't check out.

For example, if in year N youtube has f(N) new video. Let assume f(N) = cN^2. It's a crazy rate of growth. It's far better than the real world Youtube, which grew rather linearly.

But the rate of "videos that are older than 5 years" is still faster than that, because it would be cubic instead of quadratic. Unless the it's really exponential (it isn't), "videos that are older than 5 years" will always surpass "new videos this year" eventually.