Agreed, I would place the onset of 'late career' Heinlein at Number of the Beast.

Which it turns out Wikipedia agrees with, when I went to check the bibliography, so there's that.

https://en.wikipedia.org/wiki/Robert_A._Heinlein_bibliograph...

That would have been my intuitive guess--although, looking at that list, there are IMO some real stinkers in mid-Heinlein and a couple of decent books in late Heinlein. I suppose I'd be a bit hard-pressed to really draw a clear distinction between the two periods.

His later books had the self-indulgent feel that I was referring too. Self-referential, wordy, very strange approach to life.

It's obviously totally fine not to like post-juvenile Heinlein but my point is that pretty much no one would remember Heinlein for pre-late 50s YA fiction alone.

You may be right. Though not all his pre-1960 work was YA.

One more thought, even if we don't consider Stranger and afterward as "late career" it does feel like a break with prior work, YA or adult.