Nearing forty and feel (and am told) I'm doing the best work of my career. I think the same of my boss who's in his fiftys. One of the keys has been developing

Nearing forty and feel (and am told) I'm doing the best work of my career. I think the same of my boss who's in his fiftys. One of the keys has been developing deep expertise in complex subject(s) I like and keeping up with the cutting edge. Also finding a career that lets you do this.