There is a common mistake of assuming deep antiquity to many parts of later Arthuriana. Excalibur vs The Sword in the Stone is one of those. Excalibur is probably the older of the two, appearing in the Folktale-esque Culhwch and Olwen, which probably dates to the 11th century. 
The Sword in the Stone itself dates to the early 13th century, appearing first in Robert de Boron's Merlin. Robert seems to have been heavily influenced by the story of Saint Galgano's sword in the stone, which was extremely popular at the time.
The Prose Merlin, part of the Vulgate-Cycle is clarifies that it was Excalibur drawn from the stone. This is later redacted in the Post-Vulgate cycle which makes them different.
Culhwch shows evidence of drawing from the earlier poetry of Taliesin, specifically in the instance of Diwrnach the Giant's Cauldron, which seems to be a repainted Cauldron of Annwn. Caledfwlch itself appears twice in Culhwch.
The first is in a list of Arthur's possessions, while the second is in the incident with Diwrnach. Culhwch is cursed to only be able to marry, Olwen the daughter of a Chief of Giants, Ysbaddaden Bencawr.
Culhwch enlists the aid of his cousin Arthur at his ailing father's urging. Arthur agree. They storm Ysbaddaden's castle, and upon defeating him he agrees that Culhwch can marry Olwen after completing numerous nigh-impossible tasks, because upon Olwen's marriage he will die.
Not all of these tasks are recorded, but one that is is retrieving Diwrnach's magical Cauldron. After trying to appeal to Odgar and Diwrnach to peacefully give it via messengers, Diwrnach refuses, and Arthur sets out with his retinue.
After a final refusal, Bewyr siezed the cauldron, and Llenlleog, takes up Arthur's sword Caledfwlch and "swung it in a circle" killing Diwrnach and all his men. All of Ireland descended upon Arthur and his retinue, who after Arthur defeated them sailed home loaded with treasure.
Arthur returns after and lays waste to a fifth of Ireland. Ultimately Culhwch and Olwen are wed after Culhwch completes these various tasks.
Interestingly in this earliest reference to Caledfwlch, (a name which Bromwich has determined is probably cognate with Caladbolg of Irish fame, both derived from an older generic term for Sword) the sword is not wielded by Arthur, but by one of his warriors.
Llenlleog Wyddel (The Irishman) makes a few appearances, and combined with the earlier figure Lleminawc, as well as the historical king Llenneac of Elmet may account for the composition of the later Lancelot himself. Leminawc also has roots in the Celtic god Lugh.
Much of the later French material seems to have been influenced by Breton bardic tradition, which derives from a separate tradition of Arthurian tales, and may have preserved many details that the Welsh tradition did not.
Long story short, deep antiquity is often ascribed to certain facets of Arthuriana that are quite young, while others such as Lancelot are handwaved as 'French inventions" while they probably represent deeper roots than even Arthur himself.
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