what are 'salad days' ? What does 'coming of age' mean?

“Salad Days” was the title of a British musical in the 1950s or 60s. Think it was about a bunch of privileged English kids leaving the playground of university and facing the harsh reality the real world where finding a job typically meant going to see an uncle who worked in high places. Might need a bit of work to adapt for 21st century audiences. Then again maybe not, privilege never dies.

Salad days = warm sunny weather = "the best days of your life".

Coming of age is generally regarded as having reaching the age at which a child becomes an adult. Voting/marrying without parental consent/etc.

More generally coming of age means simply becoming an adult not an adolescent. It’s a common genre of books and movies especially involving some significant dramatic event.