Maybe some folks here will find my perspective informative, speaking as someone who made the transition from individual contributor to engineering manager around the age of 40.
I've worked at big companies since I graduated college. As a software engineer, I remained hyper-focused on specializing in pretty much one specific thing. I've never considered myself particularly brilliant in comparison with some of my peers, but the thing I chose to specialize in just so happened to become super-important to the technology sector about five to seven years ago. I was in the right place at the right time. I cleared my calendar for about half a year, shut out the world, and focused entirely on embodying all the expertise I had accumulated up until then into a new feature.
Now the thing I created is a critical feature in the phone you probably have in your pocket right now. As in, if I hadn't created the feature, it's very likely that a team would have been spun up in a company like Google or Samsung to create it instead.
This catapulted me into a relatively senior position, and then I found that my hyper-specialization couldn't keep me competitive among my more capable and more generalist individual contributor peers. I got to my position by pulling a rabbit out of a hat at a critical point in time, and then I (proverbially) got promoted to my level of incompetence. I can't possibly keep delivering the same super-high impact on a continual basis.
Shortly after I got promoted, my organization started to grow very quickly. Upper management was scrambling to build levels of hierarchy to absorb the growth, and they pretty much pushed me into managing a team in addition to working on some of the residuals of the technology I had built. I soon realized that I didn't have the capacity to continue designing systems and writing code while effectively managing people at the same time. I felt that I had to make a choice: Either give up management and focus on individual contributor work, or make the switch entirely into management.
Of course I had formed for myself a false dichotomy. I could always go into consulting, switch career ladders to sales or product management, or something else along those lines. But getting a taste, I was finding that I liked management. Politics started becoming less of a dirty word for me, and I felt fulfilled and useful when I successfully negotiated a mutually beneficial compromise among parties in the organization. I loved figuring out what my reports needed and helping them to achieve their goals. I loved that I could hack some code here and there and not worry at all about whether it ended up shipping in the near future, since my impact was not being evaluated by that metric any more.
And so now, a little bit past 40, I am 100% an engineering manager. It's something that I just sort of grew into, but at the same time it feels sort of inevitable if I am to stay at my current company. At my level of seniority, upper management would only be happy with me going back to an individual contributor role if I were to pull more rabbits out of hats. I suppose I've accepted that those days are probably behind me, and I'm ready for the next set of challenges.