At forty I was just getting started. I'm 63 and I have done lots of super hard projects and I keep learning more and I have advanced up the tech ladder. And, the problems are harder than ever. For me the key is going after the hardest problems because that is what I like the most. Think system software, compilers, AI. It just seems to have been a good career move though that was never my intention. I've worked in areas of the software field that are thought of as difficult, and that has helped me.