I was going to suggest 1990 was part of the Silver Age but I was misremembering when Classics came out and changed that before I hit post.

The first few years are a funny one. The shift to non-earth vehicles in 1986 (and the cartoon jumping to 2005) makes me want to divide between 1985/86. But 1985 includes everything from 1984 that an era that was (kinda) one years worth is a bit too small. The 1987 shift was the bigger one.

And thinking about it more, 1987 changed the packaging logo and added the pixelated sunburst which is a nice divide. 1989's silver/gold packaging design, new logo, grid tech specs and smaller pack-in catalogue where another major shift.

If you just considered the US G1 I'd break it as 84-86 / 87-88 / 89-90. Maybe even that with 91-93 as the extra European Era...