Do you know why from Greek to Roman he changed from a man to a boy?
actually in ancient vase painting Eros (“Cupido”, in Latin), who was considered to be the youngest of the gods and the son of Aphrodite (“Venus”, in Latin), is often depicted as either a handsome youth or as a child, as you can see at the links below.
Sculptors preferred the image of the bow-armed boy, whereas mosaic artists favoured the figure of a winged putto (plump baby).
In short, both Eros and his Roman counterpart Cupid (Latin, “Cupido” or “Amor”) appear as either a winged infant carrying a bow and a quiver of arrows whose wounds inspired love or passion in his every victim or are sometimes portrayed as a young man wearing armour like that of Mars, the god of war, perhaps to symbolize the invincibility of love.
To conclude, it’s not true that Eros/Cupid changed from a man to a boy when passing from Greek to Roman, as you say.