You are overstating what an operating system is. The core of most modern operating systems were written by a single person, or two people - including Windows.
Operating systems become large because of hardware support. They are also large now because what is considered 'operating system' has changed - from including a desktop environment to including a web browser.
The reason why OS X and Windows have large teams and year-long development cycles is because they are a complete stack of applications, not just a kernel, fs etc.
To get an OS written from scratch to boot on specific hardware with some basic functionality should be an 8-12 week job for a competent C developer.
The best thing I did in my teens was to grab x86 docs and attempt to write an OS. I thoroughly recommend it to everybody as a good project to learn system and development (along with writing a compiler and writing a simple database).
Operating systems become large because of hardware support. They are also large now because what is considered 'operating system' has changed - from including a desktop environment to including a web browser.
The reason why OS X and Windows have large teams and year-long development cycles is because they are a complete stack of applications, not just a kernel, fs etc.
To get an OS written from scratch to boot on specific hardware with some basic functionality should be an 8-12 week job for a competent C developer.
The best thing I did in my teens was to grab x86 docs and attempt to write an OS. I thoroughly recommend it to everybody as a good project to learn system and development (along with writing a compiler and writing a simple database).