Seems like you're equating superstar developers with years of experience and that hasn't been my observation. PyTorch was created by an undergrad. I've worked with senior devs that have let engineering pass them by and are now the equivalent of non-programming programmers: https://blog.codinghorror.com/the-nonprogramming-programmer/
Correct. Sr/5 is the expected terminal level for most. Both FB and G expect steady progress to that level or you get fired (if you really can't make it).
It takes building and shipping an outstanding product to achieve certain status, if we’re talking about “superstar“ status. This has almost nothing to do with years of experience. I’d even say most superstar devs achieved it pretty early in their career.
I agree that the pool of superstar devs is tiny.