Well, but he wasn't that old (by Elder Dragon standards, anyway) when he ascended. We know he's about 25,000 years old and that he was already a planeswalker when he battled that leviathan on Dominaria 20,000 years ago. So either his body reverted back to the physical age at which he ascended during the Mending OR he was stuck in the physical equivalent of the form he had assumed as an Oldwalker (if the two are any different). Neither possibility would involve him being "truly an elderly dragon". And the novel really goes out of its way to describe how old his joints feel and how tattered his wings are, so him aging uncontrollably would be the only plausible explanation for his old age.

Maybe dragons age like elves? I remember an old Distant Planes story that said something about how elves pretty much maintain their youthful demeanor right up until they begin nearing their twilight years, then it all goes old and wrinkly in a hurry. Maybe dragons have a swift drop-off point as well and Bolas had just hit that mark?

But if he was no older than about 5,000 years when he ascended, and his full age didn't catch up with him after the Mending, then why would he hit that mark already? Niv-Mizzet is a lot older than 5,000 years, for example, and he doesn't seem to be particularly elderly. And he isn't even an Elder Dragon.

Where is the age at which Bolas ascended mentioned at/hinted in? Not trying to argue I just don’t know where to look. I started around Alara and every site I try to look up his age it just says 25,000 and I just assumed he ascended later in life.

The thing is, we already have multiple instances of other elder dragons that weren't planeswalkers or aged to tatters even thousands of years after the war.

See above; we know he's 25,000 years old, and he was already a planeswalker 20,000 years ago when he battled that leviathan planeswalker on Dominaria. That last bit comes from the Future Sight novel (page 224, to be precise, I just looked it up). Bolas's age is also stated in the same chapter. So that means he can't have been older than about 5,000 years when he ascended; it's quite possible he was even younger.

Oh my bad somehow missed the part about the leviathan when reading your reply. Thank you

No problem, happy to help. What good is sitting on all those Magic books if we don't share the information?

They make it pretty clear Bolas is feeling his age in Shards. But they seem to have heavily implied in recent stories that after the Conflux Bolas got enough magic power to stem back his aging. I haven't read the Shards book in a while, but if I remember right the part where it talks about Bolas aging is from his perspective. And we all know he has a flair for the dramatic.

But they have also been very hit an miss on the effects of aging after the mending.

I might be a bit late, but for what it's worth, here's a quote fom the showdown of Alara Unboken, when Bolas has absorbed most of the Maelstrom and is stepping out of it again:

It was unmistakably Bolas, but Ajani thought he looked larger than he had been, or somehow more grandiose. He had no scars or wounds, no frayed scales on his pinions, no scruffy patches at his joints. He was smooth, sleek, a study in armored scale stretched over lean muscle. (p. 352)

So, yup, it's made quite explicit that he looks younger and fitter than before (especially if you compare it to his description on the first page), and Ajani can see it too.

I edited the list to include the retcon that removed accelerated aging after the Mending. All I needed to do was copy in the notes after all, but I guess going forward, I won't keep updating the list, at least not with new things. I'd have to keep reading this stuff to spot retcons in the first place, and that's not going to happen.

Is the change in the Jace scene considered a retcon too? Seeing as the scene changed between this year and last.

I've been wondering that myself to be honest... I think all the weird little continuity hiccoughs in the Dominaria story make it harder than ever to draw a line. I mean, Jhoira also claims in one stroy that she knew Jodah from before the invasion for example, which is BS, but I don't think Creative actually wants to establish that as the new canon. In the case of the Jace scene we at least have that new story dude's confirmation that not making the scenes match was a deliberate decision, so I guess if the intention behind it is a criterion, you could argue it's a retcon.

Any suggestions what to do with it?

Well if the new story dude's feelings on continuity really are going to be like that, then this list is going to become so long that we might need 4 posts because we will hit character limit for a single post. So I'm really hoping that isn't the case.

I think the best answer is create a separate list for this story's issues, because one I think a lot of the little things are because the author didn't really KNOW all the stuff she probably should have (like, Ethan went through and read ALL the things and made an updated map comparatively) and some of it does qualify as a retcon (like the Jace thing)