Ok I all this info actually clears things up quite a bit!
1) I had been calculating my dates based on the exact day of approval. Most of my dates were somewhere around the 25th-30th of the month. I've updated everything to be the 1st of the month. This effectively shifts everything by 1 month (see below).
2) I use the date difference function in excel "datedif". For illustration it gives the following results:
1/12/2010 to 1/11/2020 = 119 months
1/12/2010 to 1/12/2020 = 120 months
1/12/2010 to 1/13/2020 = 120 months
... this means I am effectively rounding down. Is this the desired behavior?
3) I was able to find the reason codes for some historical numbers! My FICO 08 score was around 778-791 since August 2019 until September 2020 when it jumped to the 820's. I was able to find info from both Discover and Experian with codes both before and after the increase. For Discover I'm looking at 8/21/20 vs 9/21/20. For Experian I'm looking at 11/3/19 vs 4/2/21. They are both in complete agreement for the codes before and after the jump. The results are:
Before the increase:
+ no missed payments
+ revolving utilization is low
- age of accounts is low
After the increase:
+ no missed payments
+ revolving utilization is low
- You have no negative factors impacting your Score.
For the "age of accounts is low" description it says:
Experian:
You have a short credit history.
Your oldest account was opened: 5 Years, 2 Months ago (as of 11/3/19)
FICO High Achievers opened their oldest account 25 years ago, on average.
Average age of your accounts: 3 Years, 6 Months (as of 11/3/19)
Most FICO High Achievers have an average age of accounts of 9 years or more.
People with longer credit histories who infrequently open new accounts generally pose less risk to lenders. In your case, the age of your oldest account and/or the average age of your accounts is relatively low.
Discover:
The age of your oldest account and/or the average age of your accounts is relatively low. Having longer credit histories and fewer new accounts generally poses less risk to lenders.
4) So it appears losing the "age of accounts is low" reason code explains the 30pt increase. Here is the updated table using the first of the month for each account start date:
As you can see my AoORA (and AoOA) both cross the threshold to 72 months = 6 years. Nothing else seems to cross a 6 month threshold (notably my AAoA doesn't) so this might be a clean illustration of a clean/old/thick/non-new profile aging over 72 months AoORA.
Let me know if you want me to play around with rounding differently but that may be tricky since my excel formulas are already getting unwieldy haha. Also I have data going back to the very beginning (my first CC) and thought this graph really showed the jump. It also makes me think my month calculation is off for TU vs EX.
I hate to be the bearer of bad news, but anytime you see ANYTHING positive that even looks like a reason code, discount.
FICO 8 credit monitoring sucks plain and simple, and they suppress the real reason codes if you are above a certain level like 800. If you check your DCU score report they will give you two reason codes if you scroll down, and in your case I would be stunned if the complaint weren't about age.
Literally all the scores that I have access to age is a complaint (short credit history) and my file and age metrics are older than yours for reference with the exception of the most recent account where you have done much better than I have.
That said, an oldest account of 6 years seems entirely reasonable for the next breakpoint beyond 3 years when talking segmentation metrics. I unfortunately have had too busy of a credit file to really see that transition but I was above age that when I got into the 820s the first time when my tax lien disappeared so it sounds reasonable.
Great datapoints regardless, thank you!
@Anonymous wrote:@Adidas AoOA is a segmentation factor and you are already mature, so there should not be any gains possible from AoOA.
It appears the gain was from age of oldest Revolving Account hitting 72 months, as it is a Scoring Factor. congratulations, you've added to our knowledge base!
And as far as the negative Reason code disappearing, as @Revelate said, you can't make anything from that because it doesn't show negative Reason Codes above 800, unless you check the industry option Score Codes.
@Revelate does that excel command 'rounddown' actually round down? Because Tom Quinn clarified standard rounding is used, that's why I asked.
Oh hrm, I missed that clarification @Anonymous
**bleep** I would've had a world class datapoint on that one too, right now if I just ROUND my EX AAOA is 6 years flat and I gained zilch, whereas ROUNDDOWN is at 71 months aka 5 years 11 months (71.69 if rounding to 2 decimal places). I got zip on EX FICO 8, I did get +9 on EX FICO 2, maybe now that datapoint is conflated from my 2 year spree penalty theory. See why I hate aging metrics? 
Unfortunately my youngest revolver ticks over a year on 5/1 too, which we know is bonus points on F8.
I'll just say using that ROUNDDOWN I was spot on for 2 and 5 year AAOA metrics seemingly on multiple different models. I don't know though whether it would've been affected or not as it had call it 50% chance of ROUNDDOWN = ROUND, whereas it doesn't for this new one.