The grail hero was originally Peredur/Perceval. He is present in the oldest extant “Arthurian” stories and is the most common grail hero in Arthurian literature. He also seems to have the older elements associated with him; the relationship to the Welsh Peredur and the grafting of the story from Phillip of Flanders’ life.
However, as the Arthurian corpus began to grow and develop, there were two problems with the continued use of Perceval in the grail story. For one, he was associated with an established tale involving a buffoon who persevered and gained wisdom through his own efforts. Such literature was simple and had roots deep in the legends of Europe, but did not appeal to the higher echelons of society that were reading grail stories.
The second problem was even greater than the first, that Perceval was not Lancelot. Lancelot grew in popularity from the moment he came off of Chretien’s pen in the late twelfth century, and was by 1250 perhaps the most popular hero in King Arthur’s court. However, because of his affair with the queen, one of the most popular topics in Arthurian literature, he was unforgiveably stained on a religious level. No writer dared to pen him as the pinnacle of religion necessary to achieve the grail. Instead, his inability in this one quest served to differentiate it from every other.
Instead of Lancelot as the new grail hero, a son was created. This addition to the Arthurian Corpus had several problems, however. Lancelot was famous for his absolute devotion to Guinevere, and Guinevere could never bear his child.
So another character was created, Elaine (an Anglification of Helen). As Lancelot was the best knight, it was only right that his lover be absolutely beautiful. And as Perceval was the nephew to the grail king, so it was necessary that Elaine was his daughter.
That left only the means by which the child should be conceived. It was accomplished with the same spell that Geoffrey of Monmouth had used in Arthur’s conception; a spell to alter the appearance of Elaine.
For much of the above reasoning the evidence is incomplete, but that it leads away from Galahad’s character being even as old as Lancelot (1190s) seems unavoidable. The earliest story we possess is about a naive boy slowly growing in understanding about the real world even as he grows as a Christian. In contrast, Galahad arrives at Arthur’s court a perfect knight – pure and chivalrous.
It has been mentioned that Galahad is the young version of Lancelot. I remember reading that as well, probably in the ‘Vulgate’. It is poetic license and no more, the intent is that Galahad is what Lancelot could have been if he would not have committed adultery. So is the explanation that Galahad was given his name because his father had been called that as a child. At the time of Galahad’s insertion into the Arthurian corpus Lancelot’s childhood was still being formed. The only source on the matter was ‘Lanzelet’, and it made no such claim.