Those without phones with a camera did not so it wasn't all of them.
Preparing for a mission? no difference.
Recently returned from a mission? no difference.

Maybe it wasn't all of them, but it was close enough.

Here's an example of the attitude.
A married couple in my ward got divorced. They had a 16 year-old daughter who was at my house a lot to visit with my wife. They also had a 13 year-old son.
The 13 year-old son was very upset about it. To cheer him up, a 13 year-old girl in the ward sent a nude photo of herself. She wanted him to know she was thinking about him even if they weren't close.
That's what we learned from the older sister. She also explained, as if we didn't know, that all the kids she knew sent nude selfies. She got embarrassed when asked if she did it too.