"I was very much on the mathematical side, where you probably do your best work before you're forty-five. Having passed that significant date, I thought I would do something else"

John Polkinghorne is describing the concept that mathematicians typically do their best work prior to the age of 45. He is suggesting that he has actually passed this age and is now aiming to do something else. This could be translated as an indication of maturity and knowledge, as he is aiming to move on to something brand-new and different. He is also recommending that mathematics is a field that requires a great deal of commitment and hard work, and that it is not something that can be done easily or quickly. He is also indicating that he has actually achieved a specific level of success in mathematics and is now aiming to move on to something else. This could be translated as a sign of confidence in his own capabilities and a desire to check out brand-new opportunities.