> I feel like the current trend of agile development process common in startups now somewhat works against senior devs' interests to mentor junior devs. There simply isn't time allocated for this.

If you're a senior dev, you should be gaming the system a bit to create the time needed to fulfill your role. Never let the keeners fill your time up to 100%, unless it's truly neccessary. There's always going to be some infrastructure that needs fixing, research that needs doing, refactoring that should happen, etc. And you should always have some slack for mentoring and whiteboard discussions.

I almost always start an agile sprint with a self-generated task or two already in my queue for just this reason.