some of the actors were way too old

They seemed fine to me. Have you seen Grease?

Maybe, but back then, they looked about right. Take a look at yearbooks from the 50's and the 60's. Those kids look old , possibly because of their outdated clothes and hairstyles, but also because of the times they lived in.
Now if you are referring to the actor's ages, that happens all the time when casting movies.
Personally, I think they all looked pretty much like high school kids from back in the day. That's just me.