What you write doesn't make much sense actually. Let's say the parent is of age X. If what you say was the case, then we would be having this conversation every year, for people of X-1, X-2, X-3 years that are also on HN (and of course generally on the internet).

But it's mostly 2016 that's being talked about in such way.

If a 40 year old can "start to notice" that now, then a 45 year old could have "noticed" it 5 years ago. Didn't we have older people coming to the realization you present in 2012? Or 2008? Etc.

Either celebrity deaths is something like a probability distribution where big clusters can and do happen.

Or perhaps it's mostly that most mass media stars were created in an age when TV was getting widespread use (50s and 60s and onwards) and so they approach the end of their lives as we move on.

Eh, that's not true. It's more of an issue that there are more famous people now. I'll be honest, I don't know most of the people I see pop up on sites about them dying (I suck at names), all I know is they are a star. I notice there are more are coming up in media, though.