No version really says.
In Geoffrey of Monmouth’s “Historia Regum Britanniae” Arthur is 15 years old when he becomes king. He marries Guenevere after the Saxon wars. But we are not told how long these took. Arthur is helped in the Saxon wars by his nephew Hoel. Some later adaptations of Geoffrey make Hoel to be the son of Arthur's younger sister Anna. So we should probably imagine at least 18 or 20 years passing between Arthur becoming king and marrying Guenevere if Hoel’s mother was Anna rather than supposing an older illegitimate daughter of Uther Pendragon or a daughter of Gorlois and Ygerna.
According to the “Prose Lancelot” Arthur married Guenevere early in his reign, just before the death of King Ban, Lancelot's father, when Lancelot was stolen away by the Lady of the Lake. Indeed, we are later told that King Ban was in Britain for Arthur's marriage to Guenevere, and that during that trip King Ban fathered Lancelot’s younger illegitimate half-brother Ector of the Fens. So the chronology would go:
Arthur becomes king.
King Ban fathers Lancelot.
King Ban goes to Britain for Guenevere’s wedding.
King Ban fathers Ector of the Fens while in Britain.
While Lancelot is still “in the cradle', King Ban dies and Lancelot is stolen by the Lady of the Lake.
But when did Arthur become king?
In the French “Death of Arthur” we are told that Gawain could well have been seventy-six years old at the time, that Arthur was ninety-two years old, and that Lancelot was some twenty-one years younger than Gawain, that is about fifty-five years old. According to a statement in the early part of the “Death of Arthur”, Guenevere was a good fifty years old. This of course means that Arthur was about forty-two years older than Guenevere. But this account contradicts the account in the “Prose Lancelot” that Guenevere was married to Arthur when Lancelot was still an infant. Indeed, Guenevere is now supposedly younger than Lancelot.
In Robert de Boron’s “Merlin”, Arthur was brought up by Antor. Antor makes his son Kay a knight. Now Kay have been removed from his mother’s breast so that Arthur could suckle there, as Merlin had demanded, so Kay could not be more than about four years older than Arthur at the most. And Arthur is called a boy.
There are two sequels to this account, today called the “Vulgate Merlin” and the “Post-Vulgate Merlin”, independent early histories of Arthur’s reign.
In the “Vulgate Merlin”, we have a backflash in which Arthur begets Mordred on King Lot's wife the night before Arthur pulled the sword from the stone. In the “Post-Vulgate Merlin'' Mordred is begotten as soon as the story begins, after Arthur becomes king.
There may be supposed to be a gap before the beginning of these sequels of which we are told nothing, but perhaps not.
But the “Vulgate Merlin” keeps careful track of the calendar. After defeating six rebel kings, Arthur holds court on the feast of Our Lady in September. The following Easter Arthur comes to the aid of King Leodagan, the father of Guenevere and is betrothed to her. At the end of May, Arthur returns to Camelot and knights his nephew Gawain, along with Gawain’s brothers Agravain, Guerrehet and Gaheriet and some other knights such as Yvain, Sagremor, and Dodinel. Athur’s forces then go to Gaul and defeat King Claudas’ host. King Ban fathers Lancelot. Arthur returns to King Leodegan’s kingdom of Carmelide and marries Guenevere there. (But according to the “Prose Lancelot Arthur and Guenevere had been married in London.) Arthur then holds court in the kingdom of Logres in mid-August. An agrreement is made between Arthur and the rebel kings to gather together on the Feast of Our Lady in September to battle the Saxons. Mention is made that the Saxons have been ravishing the land for two years. The meeting occurs, and the Saxons are utterly defeated. King Ban returns to Gaul, and fathers Ector on the way. See http://quod.lib.umich.edu/cgi/t/text/text-idx?c=cme;cc=cme;rgn=main;view=toc;idno=Merlin for a late Middle English translation.
The “Post-Vulgate Merlin” is less clear about its chronology. But Arthur is too young to have a beard when he is first challenged by King Rion. Soon after Rion’s army under Nero is defeated by Arthur, who then battles King Lot who has rebelled against him. Lot is slain by Pellinor. Lot's son Gawain, then eleven years old, vows to avenge King Lot. Merlin spends some time on a magic island. Then he returns to court and learns that Arthur wishes to marry Guenevere who has not been previously mentioned. Merlin leads an embassy to King Leodagan and returns to London with Guenevere. They are married at Camelot. Gawain is knighted at the marriage feast. Part of the “Post-Vulgate Merlin” was adapted into Spanish as “The Cry of Merlin the Wise” and mixed with other material. An English translation appears here: http://members.terracom.net/~dorothea/baladro/index.html.
In short, one cannot expect to find a congruent chronology between different work, and often even within the same work. In the “Prose Lancelot” Mordred is younger than Lancelot and appears as a boy early in the work while Lancelot is at that time a proved knight. But we are told Mordred was begotten before Arthur's marriage to Guenevere which is impossible according to the chronology. In both these Merlin tales Mordred is older than Lancelot and his elder brother were already knights before Lancelot was even born (Vulgate) or when Lancelot was still an infant (Post-Vulgate).
But in Malory, one Gawain's brothers, Gareth (corresponding mainly to Gaheriet in the French tales) is much younger than Lancelot.