There's a real danger as a senior level employee that if you go out of your way to mentor younger coworkers, you become the go-to training guy. Your coworkers stop asking their peers or their manager for help and start asking you directly. Other coworkers send the new hires directly to you so your manager doesn't even see the amount of questions you're answering. And all the while your actual workload isn't getting any lighter. And if you complain, you look like an asshole. What kind of person won't help train new employees? Why are you so anti-social? Why aren't you a team player? It's just one or two questions here and there. And when it comes to performance reviews, your mentoring counts for a hell of a lot less than the number of projects you finished on time, so now when the company isn't doing so well, your output per dollar is a heck of a lot lower than the people you trained, so you're out the door. No one sees that the new employees never would have finished their project without your help. Your name isn't on that checklist, it was their project. What do you mean you helped? We gave you two weeks to train them, why are they still coming to you for help? Did you not train them properly?
So either you get laid off, or you quickly realize that you have to start turning away questions and mentoring. I love training new employees. But that's not my job. My job is finishing projects. If mentoring gets in the way of shipping, guess which one I'm going to drop first?