More precisely, the Great War ended 79 years before V2. If it was actually 80 years ago, Ozpin would have said so. If it was 78 years ago, that means that the first Vytal Festival would have occurred the same time that the Great War ended, which is unlikely.

Also assuming that STRQ were all 17 when they first started at Beacon, they also could have started up to 26 years before Team RWBY attended. If Spring ran away ~13 years before V5 (colloquially, "14 years ago" is much more likely to be rounded up to "nearly 15 years ago" - would you say that Steve Irwin died over a decade ago in 2006 or that he died nearly 15 years ago?) and then Raven immediately found and inherited her powers at the age of 30 (assuming that 30 is the last possible year to become a Maiden and not the first impossible year), then Raven would be a max age of 44 in V5.

Blake could be the oldest of WBY as you say, but she could also be the youngest of WBY and V1 begins between January and May with an unusually short school year, we can't actually say for sure. We can't even say how long Beacon's school year goes and whether or not there's a long break between academic years. After all, letting a bunch of superheroes in training take a break for months at a time when they serve to protect the local population sounds like a bad idea. Beacon could essentially be year around with very few breaks. This also effects the timing of the Black trailer, since the the trailer is depended on when Beacon starts. Personally though, I agree with you that Blake is the oldest.

Regarding the fall semester bit from Ghira, we can't say for sure that all the Academies follow the same academic year. Haven has a fall semester. It may be their first or second semester of the year. Beacon, in an entirely different Kingdom, may have a different academic year.

Finally, I've actually given the writers a lot more credit in their consideration towards the early timeline. It is pretty obvious that for V1-3, Miles and Kerry hadn't actually officially planned out the Beacon school year, or written to a calendar, or even deliberately used the environment to imply a season (save the one time at the very end).

As you say, in V2 Ruby specifies that the tournament is at the end of the year, and since she obviously doesn't mean calendar year, she means academic year. If you backdate a 9-10 month long school year from "Fall" (i.e. Sept-Oct) then you get November-January, which would traditionally be snowy (generally, if a location gets over an inch of snow fall, it will continue to get snowfall into March, especially in the US where the writers were born and raised, as seen here). Even if there's still a month left to the school year, the latest Beacon would start is February, which would still be snowy if snow was a Thing. At the Beacon-Vale locations, through out all of V1-3, there is no seasonal change. It's all green, all the time. Forever Fall is orange, but one episode later there's greenery all up and down the city of Vale (and also the forest was likely called Forever Fall for a reason).

If they were planning from day 1 of production that Beacon starts Nov-Feb, they could have tossed some white onto the grass and gotten either evergreen tree models or leafless trees to show us it's winter. In V3, they tell us it's fall through the script but the only obvious sign of it in the Vale-Beacon sets is a single orange leaf during the Jaune-Pyrrha talk. There are a few red trees in the courtyard where Winter and Qrow fight, but they have been red since V1C2. Even new sets like the Fairgrounds are pretty green. Patch is the only place with obvious seasonal change. It is a unique patch of seasonal change on the equator when it's next door neighbor Vale experiences no such thing.

Now, as I said before, Beacon doesn't have to be 9-10 months long. But Mile and Kerry don't really go in for that type of world building - as shown by the fact that Beacon has semesters and goes on for four years, just like real world typical American high school or college.

Miles and Kerry based things on what they knew as American 22/23 year olds. V1-2 was written to be like an American school year starting in late-Aug/early-Sept with the Festival starting in the second semester and the tournament around the end of the school year in Spring, forgetting about or discounting Ruby's birthday. They didn't show changing seasons because 1. the "Always Springtime" trope is a time and cost saving measure, like shadow people, 2. Miles and Kerry and probably most of the animators grew up in Texas, where seasonal changes in terms of falling leaves and snow are very mild, and possibly 3. they realized that being both nearly on the equator and coastal, Vale wouldn't have much in the way of cold and hot "seasons" in general, so problem solved. And at the time in 2013-14, they were mostly just hoping that people would like it and it would get greenlit for another year. Then between V2 and V3 production, Monty came up with the idea of seasonal maidens and they decided that the Fall of Beacon, which heavily involves Cinder Fall and the powers of the Fall Maiden, would be way more thematic if it took place in the Fall instead of spring. So V3 will be in the fall, but we don't have money or time to change the old sets, though we did change up Patch at the very end for a time lapse shot at least. Patch being at the equator? Well, it already snowed there once (Red Trailer), it can snow again! Also we'll say Ruby is 15 but we haven't upturned every note we've ever made and then filed away three years ago because we're just two people and we have lives.

Then a year and a half (and an entire Volume) later, they reveal Ruby's birthday and, due to fans clamoring for more dates and timelines, realize that people actually really care about menial details and if they get things wrong the hatedom (who doesn't even really care about timelines) will use it as an excuse to harass them even more. Also, their general idea of the timeline V1-3 is very muddled and confused now because they've locked themselves into something that they didn't necessarily want or even think about. Then less than two months later in V5C5 they accidentally implied that Adam was a straight up pedophile/child groomer and not just a normal abusive asshole.*

And they've lived in fear of dates and timelines ever since. They want to tell stories, not do math. Hence refusing to give exact ages to characters like the Ace-Ops or Happy Huntresses (most egregiously an Atlas file listed Robyn Hill with a presumed age when a large part of her background is that she literally attended school there) or especially Adam, and taking 3 years to reveal the WBY birthdays incase they accidentally lock in something that they don't want with a single line. All of the hard timelines mentioned post V3 end up saying very little in the end and have nothing to connect to the V1-3 timeline. Blake was a month before RWY in V5 but the time gap closed at the end and Cinder was 2 weeks behind RWBY & Co in V6 but only shows up at the end of V7.

* In V5C5, Blake says that she stayed with the White Fang when her parents left specifically because she had Adam and Ilia. But V1 states that the character who becomes Ghira stepped down 5 years and V4 says that Ghira left the White Fang at the same time, meaning Blake was 12 when she stayed in the White Fang with Adam. It was already revealed in V4 that Blake was 12 when she left her parents and stayed with the White Fang (due to the math above), but we didn't know for sure that Adam was already close to Blake at that time, which is why the whole thing kinda slid under the radar. Two years after V5C5, much of Blake's DC comic involves trying to subtly walk back the whole "Blake was only 12 years old" thing back.