One was mid-50s, and nearing an early retirement. He'd worked in a dozen or more languages, was working mostly in Python and C (for performance-critical Python extensions) when I worked with him. He'd been significantly involved in the development of the IBM Oberon compiler and environment in the past, as well as done something with OS/2, though I don't know the specifics. Very broad experience, though, including a lot of low-level stuff. He was treated uniquely in the company at the time. He was the only person with an office by himself, he skipped most meetings, and was generally a bit quiet and introverted. He liked to come in, do his work, and go home. He didn't stay late, didn't work weekends, didn't do crunch-time even if everyone else was (because his part of the project was already done, on schedule), didn't go out for drinks after work, usually, though he would go out for lunch with everyone. He was a super nice guy, and generous with his knowledge, he just wasn't into being very social. He wanted to go home, be with his family, and work on his hobbies (some involved code, others were gaming and video production and 3D modeling...so lots of tech hobbies).
The other I met when he was in his mid-20s, he's in his mid-40s now, and I still work with him. He was extremely talented and productive then, and remains so today. He maintains roughly a million lines of code that he mostly wrote by himself (mostly while working an unrelated full-time job). It's a world unto itself (for better or worse, it looks nothing like any other project you'd find in the same language, and his coding style hasn't changed notably in at least a decade), but he delivers new features in an absurdly short time. He's a completely different sort of developer than the previous one I mentioned; the first was always eager to try new things. The latter has built up a system that works for him, and rarely changes...he just keeps churning out code every year at an alarming rate, because he knows the system inside and out. Bugs evaporate in seconds sometimes, once identified and reproduced.
In the former case, I was impressed with the breadth of his knowledge and his ability to drop in on a project and figure it out extraordinarily quickly, and his ability to zero in on what needed to be solved today and what would fall out naturally and easily once the core problems were solved. While in the latter case, I've always been impressed by how quickly he can deliver pretty much anything the project needs. He's written everything from the ground up (including things most people would pull in via libraries)...so, no element of the codebase is a mystery to him. He's looked at the same codebase nearly every day for two decades and he can sit down and be productive immediately.