(Source: Life of Saint Efflam)
So I found this is a rather eye-opening statement from the hagiography of Saint Efflam, that 450 A.D. is the time of Arthur's crowning.
By that logic, Arthur was born in 435 A.D., since traditionally Arthur assumes power when he's around 15 years old.
But going further, there are two significant points in the story of Arthur's life: the Battle of Badon Hill, where Arthur decisively subdues the Saxons, and the Battle of Camlann, the final battle with Mordred.
To start: Annales Cambriae, where the earliest mention of Arthur and Mordred is found, actually dates the Battle of Camlann to the year 537 A.D. (other manuscripts say 539 A.D.).
This would mean Arthur lives to a ripe old age of 102 years old.
The thing is... it makes a kind of sense. Because in Vulgate Cycle -Mort Artu, we get a statement of Arthur, Gawain and Lancelot's ages:
(Source: Vulgate Cycle - Mort Artu)
It is off by 10 years but, damn, it is close, so margin of error. Then again, this statement was made during the War against Lancelot. Vulgate narrates that following this, the War against Lucius Tiberius and Mordred's Rebellion ensue right afterward, so that may account for the missing ten years?
Also, this info from Mort reveals the relative age gap between Arthur, Gawain and Lancelot. Gawain being 76 y/o means he was born roughly a year (or so) after Arthur's coronation, ~451 A.D. onwards.
This leads into a bit of weirdness regarding Vulgate Canon, since the Orkney Brothers (sans Mordred) in Vulgate Cycle start their Knightly Careers and defend Camelot while Arthur is on an expedition to Cameliard, defeating King Rions and courting Guinevere. Arthur's critical battle with King Rions - which is just after his shotgun wedding with Guinevere - is said to happen when Arthur is 28 years old:
This also means, while Arthur married Guinevere and defeated King Rion for Leodegrance in around 463 A.D., it leads into a very weird conclusion that Gawain was around 12 years old around this time, when he started fighting the Saxons (with his even younger little brothers!) in Arthur's absence. If we add 10 years to that number (adjusting for the statements in both Efflam and Annales), it becomes the more believable 22 years old for Gawain.
Moving on to Lancelot, the age gap between him and Gawain is 21 years while the age gap between Gawain and Arthur is 16 years.
This means Lancelot's birth was around Arthur's 37th year. Extrapolating this to Efflam's statement, Lancelot's birth year would be 472 A.D. around 22 years after Arthur's coronation and 9 years after marrying Guinevere.
Vulgate Cycle states that Lancelot goes to Camelot and knighted when he is around 15-18 years old (it's inconsistent), which would reasonably be the age of maturity. This means Lancelot begins his knightly career in the 490 A.D. at earliest and would meet a 55 year old King Arthur.
The last consequence that Life of Efflam's line implies is the timing of the Battle of Badon Hill, Arthur's decisive victory over the Saxon encroachment. Multiple scholars have speculated when Badon supposedly took place, with estimates going from as early as 482 A.D. to as late as 516 A.D.
If we presume, for the Big Battle of Arthur's entire career, is around 500 A.D., Arthur is 65 years old, Gawain 49 years old and Lancelot 28 years old.
The amusing and romantic conclusion about all of this, is that young freshly knighted Lancelot meets the Great King Arthur for the first time, at the midpoint of Arthur's life, just right before Arthur's biggest military achievement, one that secured Britain's peace for a couple of decades. (With Lancelot maybe even making his big debut in Badon!)
And as a result, the next 20-30 years or so afterward, would be everything in Lancelot's adventures - Galehaut, Morgan, False Guinevere, Meleagant, Galahad and the Grail Quest, etc.
Of course, the BIG problem with both Vulgate and Efflam's statements is that, according to Bede, Vortigern's entire drama with Hengist and the Saxons takes place around that 450's time period. Dates given in Bede's chronicle state the Saxons began arriving anywhere 446 A.D. to 449 A.D.
This makes Arthur's coronation at 450 A.D. impossible, as traditionally, the entire story of Aurelius Ambrosius and Uther Pendragon needs to unfold, especially the magic conception of Arthur as well as the fact that Arthur never meets Vortigern.
Even if we adjust to saying 92 is the oldest Arthur ever got with the Annales 537 A.D. date, that still leaves us with Arthur being born at around 445 A.D. and coronated at 460 A.D. - still far too close to Vortigern's era.
