Hmmm, eras for Ayu are decided in terms of... musical style, the context to which Ayu writes her lyrics, and probably to a certain extent her sales.
It was quite clear that since I am..., Ayu's lyric-writing style changed from during Duty, so I suppose the first era's ending can be marked with the release of A BEST.
Personally, I would count I am..., RAINBOW and A BALLADS as its own era simply because in Memorial Address we saw Ayu really start to get into the rock genre.
I wouldn't count her previous ventures because they were still highly themed (like the 'rock' tunes on RAINBOW, or say, something like NEVER EVER.)
Memorial Address - A BEST II can be its own era. XD This time it's more to do with sales and also Ayu vocal-wise.
Secret - A COMPLETE. How does that sound? A little longer than Secret - GUILTY. This era is the "Ayu has deeper vocals but doesn't venture into NEXT LEVEL yet" era. I wouldn't group NEXT LEVEL in with the Secret era.
Honestly, the era thing is really subjective. You can count Ayu's "foray into Asia" as its own era.