You'd be surprised at what juniors can pull off. I have seen fresh-out-of-college new grads write performant GPU kernels that are used in real world library implementations for particular architectures.

Of course they can. It doesn't take that long to learn CUDA/etc and hardware details, read through some manuals and performance guides, do some small projects to solidify that knowledge. What you need is talent, and some months. That's why at university I saw plenty of students pull off amazing projects, and there's nothing eyebrow-raising in the slightest about starting a PhD a year after getting a bachelor's and writing software more sophisticated than most programmers would write in their career.

I think the programming profession overvalues experience over skill. However, when I was young I had no appreciation for what the benefits of experience are... including not writing terrible code.

Most of the juniors I've worked with would make numerical errors and give up/declare premature victory before getting the implementation to a robust state. I'm sure there are exceptional young folks out there though.

  > Most of the juniors
Most senior programmers can't write CUDA kernels either. Even fewer can write ones that are any good.

What's your sample size?