And for some reason, we still ‘dial’ them.
As for ‘dude’: I’m 47, and have used it most of my life. Yet when I use it as a personal pronoun to my 14-year-old nephew (“Dude, you can’t be serious!”) I get a sidelong glance.
Kids these days…
When I was a child being happy was gay
When I was an adult, being homosexual was gay
As I approach old age, being mediocre is gay
The verb “mock” has made a comeback in the last few years.
When I was a kid, we would never have used that word. You “made fun” of somebody.
When I first heard a kid complain that they were being “mocked,” it sounded very strange and archaic. Now I hear it all of the time.
Does anybody still use “word”?
Or “word up”?
The answer to the OP is “When it ceases to be relevant.”
Some slang gets assimilated into mainstream language, i.e. “OK”, others maintain the status of slang (“fuck”). Both examples are really old and still serve a purpose, whereas “groovy” doesn’t.
This is My Happening and It Freaks Me Out!
Just pop a couple of the Red pills.
They’ll mellow you right out! 
Here’s some Happening music that might help too.
“23 skidoo” is an interesting example, as presumably it never really meant anything, unless Illuminatus! is on to something of course. It is the kind of slang that has no content, just like “Kilroy was here” and “all your base are belong to us”. They are used only to show that you are in on the joke, so once everybody is in on the joke the joke is over. This is a problem that all slang faces, but it seems to me that slang words can transform meaning or become mainstream easier than fixed expressions with unclear connotations.
That’s gas!
Almost every day.
Don’t know if this is appropriate for this thread but I tend to use “you’ve been done with the rough end of a pineapple” for someone really arsey (lucky), or for a bad run of luck I say “I couldn’t shit in the ocean without splashing the sides” or “I couldn’t shit up a downpipe” (all of these are Nanna-isms). Also, I still call people ‘china’ as in china plate which rhymes with mate. Other slang we use is ‘koff’ for ‘fuck off’ and ‘cankles’ which is a nasty name for a girl who is chubby so her calves meet her ankles without definition.
I used “copped the rough end of the pineapple” not twenty-four hours ago, but that’s the sort of ocker cove I am. Now, I hope hope your chooks turn to emus and kick yer dunny down. 
Strewth!