Please don't interpret this as a defense of Robert Heinlein's books as it has been decades since I read them and it sounds like you might be more familiar with them. But weren't all the women in Stranger in a Strange Land older than Michael Valentine Smith?

Are you confusing the character with Lazarus Long from Time Enough for Love, et al? I think he was thousands of years older than some of his (adult) love interests...

That's a rather puritan and skewed perspective. Afaik Stranger in a Strange Land is a very weird book but very tame and not pornographic at all. And it was an important artwork that explored the themes of the 60s sexual revolution. A time where people knew very little about sex and gender and questioned everything.

What? He gets freaky in some other books, but Stranger is pretty clean for a free love manifesto. Every character I can think of was a full grown adult, I don't think there's a single scene even euphemistically depicting an old man sleeping with a young girl. All the sex even referred to was between people of relatively close age.

Grok just means to understand/drink fully. You drink water, and it becomes a part of your body. You understand an idea, and it becomes part of your mind. It's an efficient word to communicate "understanding something fully enough to functionally incorporate it into your worldview". It's not that intellectual.

I dunno. I only ever used it as shorthand for being able to understand the basics of something. Also, words can change for the better. So in this case it went gross -> fine -> gross.