He knew she had grown-up things on her mind.

I just thought it was the most grown-up thing I'd ever seen.

"It seems an awfully grown-up thing to do," she said.

A. To me, it was grown-ups talking about grown-up things.

But she seems used to doing the grown-up thing, and staying in New York isn't too bad.

This is a pretty grown-up thing we have to do.

I didn't explore rock and then move into jazz as some kind of grown-up thing.

"I can talk about grown-up things too," Bill said indignantly.

"Wouldn't that have been the grown-up thing to do?"

Children could see what it meant to be an adult because they were surrounded by adults doing grown-up things.