Being a summary of the most widely recognized formal period classification of Warhammer Fantasy Battles (WHFB) and Age of Sigmar (AoS), used for both forums and Facebook groups:
Editions 1-3 of WHFB are Oldhammer (1983-1992), roughly the era of Bryan Ansell.
Editions 4-8 of WHFB are Middlehammer (1992-2015), the era of Tom Kirby.
All editions of AoS are Newhammer (2015-), the era of Kevin Rountree.
I would do the division differently =>
Editions 1-3 of WHFB are Oldhammer
Editions 4-5 of WHFB are Middlehammer aka Herohammer
Editions 6-8 are Newhammer
everyone knows the only way to define eras is logo
Tend to agree with @Admiral âs classification although Herohammer is a distinct subperiod. Another interesting question is whether you can apply similar categories to 40K where the transitions are more fluid. To me, Rogue Trader has a distinct feel to 2nd edition, yet they have similar aesthetics. After that, you have the grimdark Middlehammer era until perhaps the advent of the Primaris marines.
gold logo - i am child
black logo - i am teenager
shit logo - ignore
play gold logo game with black logo era minis
ignore shit logo
Helms oldhammer
Hats middlehammer
Masks new hammer
I think that is the only really clear definition! 
Good takes on it, everyone. @Zanko 's take is elegant and makes good sense. And headgear can always be trusted. Although now Iâm curious about the logos. Whatâs the shit logo? 
There is a practical reason I posted this: There are dedicated forums and Facebook groups to Oldhammer and Middlehammer. While Oldhammer is crystal clear, Middlehammer is less so. Is Middlehammer GWâs red period in the 90s, or more than that?
Knowing that everything from 4th to 8th editions WHFB is welcome on the Middlehammer sites means you can post all relevant stuff of your own there, should you feel like it.
Cheers
I have to say that I have a problem with the definition that AoS is Newhammer. Because in my eyes, AoS is not Warhammer and therefore not Newhammer. Itâs just âonlyâ AoS, which is not meant in a derogatory way.
Yes, itâs a different setting and wargame all together. Personally I find your periodization the best, but here I mostly chucked it in because it fit the schema with periodization coinciding with Games Workshop CEOs, and the lack of often-spoken-about Newhammer after Middlehammer (meaning dedicated sites). It looked untidy with Old and Middle but no New. 
A caveat on AoS: Itâs not Warhammer Fantasy (the original Warhammer), but itâs still Warhammer: Age of Sigmar in the same way that Warhammer 40â000 is Warhammer even though not obviosuly Warhammer Fantasy. All three settings share Chaos and at least 40k and WHFB share Eldar/Elf gods.
You are right about the âWarhammerâ, I have expressed myself in a misleading way. For me personally, âWarhammerâ = âWarhammer Fantasyâ, thatâs how I was socialised! 
I 100% agree with esteemed longbeard zanko
Yeah, same here. If I hear Warhammer I think only of Warhammer Fantasy. 
Because it was the original name for the original game before 40k became more popular and one had to distinguish WHF as opposed to the other Warhammer one might think of.
To my observation Zankoâs list is the closest. Ofc. one doesnât say âNewhammerâ if there is no need to distinguish it from some (possible local) majority, but names like âMiddlehammerâ for the Herohammer editions 4th/5th definitely have some spread.
On the other hand, if one wants to speak about WHF as opposed to AoS, this game has a name. Never seen AoS being called Newhammer or something, it has several editions on its own, from the 4 pages of do-what-you-want with dance-around-the-table-and-make-funny-sounds warscrolls, to the 40k (8th+) testing grounds it kind of is now.
In 40k terms like Oldhammer are also not used because they refer to Warhammer as in Warhammer Fantasy.
40k splits in:
as far as rule mechanics and design ideas go. But they donât have funny names. HH as 30k at most.
Eighties hammer: 1st ('83), 2nd ('84) and 3rd ('87)
Nineties hammer: 4th ('92) and 5th ('96)
Noughties hammer: 6th ('00), 7th ('06) and 8th ('10)
I thing Zanko is closer, but for a long time Iâve been seeing Oldhammer used for everything from 1st to 5th edition, even if 4-5th did have a different look and gameplay to 1-3rd.
@Shef may be joking but I think itâs actually accurate:
1-3 old
4-7 middle
8 ânewâ (or just WHF)
There is only one âcurrentâ (if technically dead) edition so it doesnât make sense to me to lump it in with 6th and 7th.
The alternative is a rules-based distinction, which breaks it into 4 sections (1-3, 4-5, 6-7, 8), but that doesnât fit in a nice old-middle-new distinction.
Many good ways of classification here. A delight to read. I agree that three periods of WHFB makes the most sense, both stylistically and rules-wise. With 8 being a reasonable stand-alone candidate, with its own dedicated forum: EEFL.
But remember: For posting Oldhammer and Middlehammer on their dedicated sites, itâs 1-3 and 4-8 thatâs the name of the game.
I donât know the specific sites youâre referring to, but in the olhammer fb trade groups 2000 is usually the cutoff year for miniatures (in addition to being OOP).
I was not joking. our fantastical head gear has been the driving force behind defining eras in Warhammer.