I said about 70 years old at the time they launched their ships after the Protostar into the anomaly (and just worked off a rounded number).
I kept in mind the premise that you mentioned before the Diviner might have been in his mid 20-ies while FC happened... in which case, if he was approximately 25 (or even 28) years old, then that would make him 77-80 years old by the time the order went after the Protostar. Add another 20 odd years for when he lands on Tars Lamora (because we know he was searching for the Protostar for years before Gwyn was made), and you get roughly 97-100 years old by 2383.
Ascensia (if she was of same age as the Diviner or just maybe 3 years younger) would have ended up being 77-80 years old by the time we see her on the Dauntless in 2383 (she was in that time frame for 3 years).
Its possible the Vau'N'A'Kat do keep well up until say 80-odd years and then start to age more rapidly.
Yes, or the Diviner could have gotten a lingering illness which severely weakened and maybe prematurely aged him. When he finally left the tank after all those years, he seemed much more healthy again, even without his special suit during his time on the Dauntless. I used to think it was weird that the Diviner seemed to be so frail, just 3 years after leaving the time anomaly, while Ascencia (exactly the same age in the present as he was when he had Gwyn created because of his frail health) seems to be almost a spring chicken.
But looking at what we think their ages actually are now that they seem to come from 52 years in the future (and First Contact was possibly in 2384, otherwise likely in 2385 and maybe 2386 at the latest given that Starfleet is said to have scaled back deep space exploration after the Mars disaster and the loss of many vessels and maybe personnel), it's Ascencia's apparent (relative) youth that is surprising. And it isn't merely the genetic implant; she looks just as good (or better) in the flashback scenes of "The Order", especially when she is depicted on her vessel in the anomaly.
So I guess the Vau N'akat do have pretty long lifespans, at least in terms of not having severe symptoms of old age as quickly as we do (now).
Something that wasn't quite made clear in the episode, is what exactly transpired between Gwyn and Starfleet. The issue of what kind of support she will get has been discussed, but I wonder whether Gwyn could have entered Starfleet if she wanted or if Starfleet was hesitant to accept her (as Vau N'akat who has shown conflicting loyalties at times, the brass may not be entirely sure about her).
Janeway seemingly implied she was only able to convince Starfleet to accept the other 5, and Gwyn seemingly implied she deliberately chose to go to Solum. The answer may be in between: as soon as Gwyn made it known that she wanted to go to Solum, Starfleet admirals probably thought of first contact implications (in the more juridic sense, I suppose, in that there are relatively strict rules governing that) and didn't want to have a backdoor first contact by having a Starfleet cadet or even "warrant officer in training" go to Solum, even if she would make contact only on a strictly personal basis. I suppose the latter is the case, and she isn't going with a mandate to make first contact in the name of the UFP. It's also possible though, that it was Starfleet interrogators who first proposed to Gwyn to go to Solum, assuming the admiralcy is also worried about what happened with that particular first contact (and in that case, did they postpone it until further notice?).
The question here is also if they offered Gwyn an alternative should things go badly on Solum, or she otherwise wanted to come back to Earth. Did they offer her asylum in that case? Was she assured there would be a place for her in Starfleet, if need be? It's unclear, just as it isn't quite clear why Gwyn did not confide in any of her friends, not even Zero (Dal could maybe have been difficult considering he would likely want Gwyn to stay with him, but then he could have opted to join her, I suppose, even if only in case he wouldn't be allowed into Starfleet).
In practical terms though, allowing Gwyn to go to Solum means they are effectively allowing her to divulge the existence of the UFP. And I wonder if, contrary to what may be popular belief, making publicity for the wonders of the UFP may be contraproductive if her goal is to avoid the civil war (rather than the even more ambitious goal to have Solum join the UFP). After all, the promise of the UFP apparently divided the population (or in any case created two roughly equally powerful blocs opposed to one another) and even the ones who were pro-UFP turned bitter when they found out Starfleet wouldn't support them (past a certain point).
So it would seem important for Gwyn to impress this on the (young) Diviner and other potential leaders of what would be the pro-UFP faction: that the UFP may be great and all, but they don't take sides in internal conflicts of non-members so don't count on their help and keep this in mind when dealing with political opponents who are potentially willing to use violence to stop membership or cooperation or whatever.
But I wonder if Gwyn even knows that much about the UFP and its prime directive, and other relevant protocols. That's why it would be better if she has at least been extensively briefed before she left, or she if she is going with a crew with at least one relevant advisor among them.