Young developers can be, and often are, highly productive but software development is more nuanced than just raw lines of code. Sensible design is both very difficult and very important and I don't think anybody gets very good at it with less than 10 years of experience.
The first 100K lines or so are easy and somebody with talent who is fresh out of school is going to get to the finish line at least as fast as a 10 year veteran. The real test is the next 100K lines of code which the experienced designer is going to breeze through while the novice is about to learn an important lesson: Writing large programs is much harder than writing small ones.
You're missing the point, I think. Higher-level languages just reduce the number of lines before you start to hit the maintainability wall from poor design. So, while maybe it's >100K lines of C that's where it typically happens, it might only be 10K or 20K lines of Ruby.
The first 100K lines or so are easy and somebody with talent who is fresh out of school is going to get to the finish line at least as fast as a 10 year veteran. The real test is the next 100K lines of code which the experienced designer is going to breeze through while the novice is about to learn an important lesson: Writing large programs is much harder than writing small ones.