Movies from 20 years ago will make you feel old? They make me feel young. Movies from 50 or 60 years ago make me feel old (although they were much better movies overall).
It's due to a psychological mechanism of the human memory.
Films like "Finding Nemo" are first brand-new, and are then later stored under the rubric "no longer brand-new, but still new") for a lo-oong time. (There are no separate categories for "only 3 years old," "only 7 years old," etc.)
It's only when they finally round the 20-year mark, and someone draws your attention to that fact, that your brain actually has to re-classify them.
In contrast, Re-classifying a 50-year-old movie as "suddenly" 60 years old is not a biggie.
Sort of like observing young children, who are suddenly driving cars and working as policemen or doctors - that's a big shift. But those same policemen and doctors in their mid-20s later being 40-year-olds is simply not as momentous a re-classification in our minds.
Regards,