He is right though - I was really into messing with kernel development a few years ago, and despite being an experienced C/C++ developer and working on it on and off for a couple of years, nothing useful came out of it.
You really need to be extremely familiar with computer internals, have a very good grasp of the instruction set architecture of the platform you're targeting, have expert C knowledge, and at least be very comfortable programming with assembly.
And even then you're going to fail unless you're working with a large team - there's just too many parts of a modern operating system for one person to ever tackle. Even if you only want to support a single set of hardware (one graphics card, one network card, etc), you'd spend years just writing drivers for everything and by then the hardware would be long obsolete.
I'm not trying to say that he shouldn't do it - it is great for learning how computers work on a low level - but he really does need lots of experience, and should go into it with the correct expectations.
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).
You really need to be extremely familiar with computer internals, have a very good grasp of the instruction set architecture of the platform you're targeting, have expert C knowledge, and at least be very comfortable programming with assembly.
And even then you're going to fail unless you're working with a large team - there's just too many parts of a modern operating system for one person to ever tackle. Even if you only want to support a single set of hardware (one graphics card, one network card, etc), you'd spend years just writing drivers for everything and by then the hardware would be long obsolete.
I'm not trying to say that he shouldn't do it - it is great for learning how computers work on a low level - but he really does need lots of experience, and should go into it with the correct expectations.