Do you think it hurts less on an infant?
I read somewhere that it was once (still?) believed that the infant felt no pain. I doubt very much that that's true.
I don't know. Whatever is said will be argued. I had painful procedure(s) as an infant small child and don't remember. An infant doesn't have the anticipation like the older child which surely involves some degree of anxiety of turmoil.
An infant may be more sensitive to pain; an older child may be more sensitive to pain because there is a larger area, therefore more nerve receptors.
How can you measure the pain of another? You assume there is pain.