I am the Kat that Walks By Itself

the epigram (which I wish I could remember properly) about if a young scientist says something can be done they are usually right, while if an old scientist says something can't be done they are usually wrong.

I wonder if you were thinking of Clarke's First Law which, according to Wikipedia, is: "When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."