When Seth had lived 105 years, he fathered Enosh.
We have here all that the Holy Ghost thought fit to leave upon record concerning five of the patriarchs before the flood, Seth, Enos, Cainan, Mahalaleel, and Jared. There is nothing observable concerning any of these particularly, though we have reason to think they were men of eminence, both for prudence and piety, in their day: but in general, I. Observe how largely and expressly their generations are recorded.
Commenting on Genesis 5:6-20
And Seth lived an hundred and five years, and begat Enos. Not that this was his firstborn, no doubt but he had other children before this time; but this is only mentioned, because it carried the lineage and descent directly from Adam to Noah, the father of the new world, and from whom the Messiah was to spring; whose genealogy to give is a principal...
And Seth lived an hundred and five years, and begat Enos: (d) He proves Adam's generation by those who came from Seth, to show the true Church, and also what care God had over the same from the beginning, in that he continued his graces toward it by a continual succession.