Is correct to use the adjective "young" for objects? For example, in a sentence like this: "This painting is younger than that one.", I think it would be better to use "new" for "painting", but then, using the comparative form "newer" doesn't seem appropriate either. So, my question is: Is "This painting is younger than that one." correct?
To answer the question, yes it is still correct. However it can be more ambiguous than "newer"
If you want to explicitly reference the chronology of things, using younger can sometimes be ambiguous. For example:
Her recent paintings lack the depth and allure of her younger works.
In this case you can see that younger is referring to something further in the past, not closer (and is interchangeable with "older" strangely enough)
Using "most recent" or "newest" is probably the least ambiguous way to explicitly communicate that the painting has no peers created after it was.