actually arnold could probably benefit from some kind of eyelift but it would seem maybe he doesn't want to look constantly surprised like his good friend sly.
As you can see in this pic due to age the skin above the eyelid has sagged from loss of elasticity, so it hangs down and the less open his eyes are the more slitted they can look.
Obviously in small images or further away they can look more slitted giving the oriental look.
due to plastic surgery or genes?