If you’re not a liberal at twenty you have no heart, if you’re not a conservative at forty you have no brain.
This famous saying is attributed to Winston Churchill (although it may predate him).
I saw this quoted recently on an online forum, in a discussion about changing one’s mind. I could only think of a few examples of major issues on which I have changed my mind. But there were a lot of examples in which I had come to appreciate the nuances of an argument and realized that my earlier position had been one-dimensional.
This realization inspired my version of the 20/40 aphorism:
At 20 I believed in simplistic answers and held firm convictions, at 40 I understood nuance and was more willing to listen to multiple sides of any position.
May need a little work to make it a snappy quote, but I like it.