I had thought that young venomous snakes have round pupils even though the adults have the narrow vertical slits. During our bird walk yesterday, our leader, who is a naturalist, said that even the young pit vipers have vertical eyes. Because he is a naturalist, I have no reason to doubt his word, but one of the fallacies people make is to rely upon authority; hence, I pose that question here.
A couple of data points:
Wiki says " Many nocturnal snakes have slit pupils while diurnal snakes have round pupils."
This site says"It is NOT true that you can tell a venomous snake by the shape of its pupils"
Snakes pupil shape seems to be related to their foraging behavior. Those that ambush their prey and hunt at night tend to have vertical pupils; round pupils are more characteristic of diurnal snakes that actively seek out and pursue their prey.
Overall pupil shape doesn’t change with age, so a young pit viper will have the same shaped pupil as an adult. But, of course, pupil shape changes with light conditions. Even slit-shaped vertical pupils can look round (or roundish) in low light conditions.
Also, your generalization is incorrect - not all venomous snakes have vertical pupils, and not all vertically-pupilled snakes are venomous.
I got here too late to say much of anything new. TriPolar and araminty have it covered. Except that you really don’t want to be close enough to an unidentified snake whose venomous status is questionable to be evaluating the shape of its pupils. Even if pupil shape told anything important.
Reported for move to Cafe Society.
What? This isn’t about ingredients?
Never mind.
Quoted for truth. By the time you can tell what sort of eyes it has, you’re either on the way to the hospital or laughing about how glad you are it’s not poisonous. 
I’ve never heard of any sort of creature whose pupil shape changes with age, once the eyes have fully developed. Iris color, yes,but not pupil.
I’ve given my reasons before on this Board for why I think many snakes have slit pupils – essentially the same reason cats do. I note, in passing, that the same slit - nocturnal/ round-diurnal dichotomy seems to hold for geckos, as well. Many of them have slit pupils with interesting “notches” in them.