Charles H. Spurgeon on Psalms 110:3

The dew of thy youth. There does not, indeed, appear to me any reason to doubt that, in this place, David extols the divine favour displayed in increasing the number of Christ's people; and hence, in consequence of their extraordinary increase, he compares the youth or race which would be born to him to the dew. As men are struck with astonishment at seeing the earth moistened and refreshed with dew, though its descent be imperceptible, even so, David declares that an innumerable offspring shall be born to Christ, who shall be spread over the whole earth. The youth, therefore, which, like the dewdrops, are innumerable, are here designated the dew of childhood, or of youth. John Calvin.