The Wrong Way
An apostrophe should not appear between the numeral of the year and the added “s.” Decades don’t own stuff, so why you making it possessive? Omit. Omit. Omit.
WRONG: 90’s
WRONG: 1990’s
WRONG: I grew up in the 1990’s and loved MC Skat Cat.
WRONG: During the 90’s I loved to walk to the store and buy mass amounts of AirHead candy.
WRONG: You might hate the 90’s but that was my jam!
The Correct Way
When speaking about a certain period of time with multiple years included in it—say, 1990 to 1999—then add an “s” and omit the apostrophe.
CORRECT: 1990s
CORRECT: If I could travel back in time, I would go to the 1930s for sure.
WRONG: If I could travel back in time, I would go to the 1930’s for sure.
If you want to create a contraction (OMG one of the things apostrophes are used for! *squeal*), then you need to add an apostrophe to the beginning to hold the place of the omitted portion. (1990 à ’90s) And remember, the apostrophe should hug the missing numbers/letters, not those remaining.
CORRECT: ’90s
WRONG: 90’s
CORRECT: The ’20s were a hell of a time to be a lush.
CORRECT: The hairspray industry loved the ’80s because you weren’t allowed to have bangs shorter than four inches tall.
The good thing: he will continue to write decades wrongly - so exposing his stupidity to everybody. Like he refuses to learn anything since his "20's", which means his "knowledge" is fixed since.