Are GLOCK magazines interchangeable across all generations?

No, not all GLOCK magazines are interchangeable. There are some generational differences with GLOCK magazines. They have come in basically 5 generations, and this is a breakdown of the relevant differences.

GENERATION 1 MAGAZINES
These magazines are notched on the right side for the magazine release. 

GENERATION 2 MAGAZINES
These magazines are also notched on the right side for the magazine release. 

GENERATION 3 MAGAZINES
These magazines are notched on the right side for the magazine release, and can include an additional notch in front of the magazine for ambidextrous magazine release models.

Generation 1, 2, and 3 magazines will interchange as long as the magazine release is in the standard right hand configuration (except ambidextrous models).

GENERATION 4 MAGAZINES
These magazines are notched on the right and left side (left retention notch) for the reversible magazine release, and can also include an additional notch in front of the magazine for the ambidextrous magazine release. 

Generation 4 magazines will interchange with all previous GLOCK generations. 

GENERATION 5 MAGAZINES
These magazines are notched on both sides for the reversible magazine release and  include an orange follower, raised feed lip, and a floor plate that is slightly extended and curved in the front. 

Generation 5 magazines will not interchange with any previous GLOCK generations.