When did you realise it wasn't, 5?
Rover
Some time in the 1970s I discovered descriptivism - at school, and even at university, prescriptivism ruled. I was a prescriptivist myself when I started TEFLing in 1968, proud that, unlike some of my less well educated colleagues, I at least was able to maintain standards.
Slowly it began to dawn on me that some of what I had been taught was completely artificial, rules invented by people at various times almost on a whim. The apostrophe in
1950s was one of the first things to go in punctuation. Some of the grammar rules were too firmly drummed in to go - I'm still an
I shall/you will and
if he were here person, and I have been known to use the occasional
whom, but at least I no longer insist that my learners produce them.
The apostrophe in
1950's doesn't worry me much. I tell learners that it is not necessary, but I don't worry too much if someone uses it - there are more important things in life to worry about.