> If you can find a startup that succeeded by hiring new grads and junior engineering first, I'd love to read about it.
Mentoring is not something that's only done for new grads and juniors.
Wherever there are skilled people in the company interested in learning more about the systems around them and how to work with them well, it's generally a good idea to figure out some way to mentor/grow them. Whether officially, nor unofficially.
For non-technical roles it's likely a good idea to do the same in ways that suit there as well.