Is it just me or I find it difficult to believe that 19 year olds can implement the LLVM alternative?
They exist, and have truly enviable amounts of time for projects.
Also, don't overestimate compilers (or kernels). They can be much simpler than they might seem! The difficult/tedious parts are optimizations(!) and broad compatibility with bizarre real-world stuff.
(But you probably need to be older to be a good project manager.)
Why though, it seems with time we learn a LOT more and then there are 100 ways of doing things and get into analysis paralysis. The passion hasn't been the same as well.