No apostrophe is required with the 's' for decades when listed in full (e.g. 1990s). In situations where just the decade is indicated (e.g. '80s) the direction of the leading apostrophe indicates that information ahead of the number “80” has been removed (i.e. it is a contraction of 1980 in this instance).
- 1990s
- 1840s
- '80s
- 1960's
- 20's