Is it reasonable for me to teach my seniors technical skills?

Is it appropriate for others within the group to expect me to teach them the technical skills I acquired on my own to help me do my job better?

If you were hired to do so then yes, it is expected from you. However, it seems that you were not hired to specifically teach your coworkers about programming languages whatsoever, so technically you are not forced to do so.

However, have in mind that helping others in your company is something valuable and to some degree expected from you (some people call it "being a team player"). It is always expected for coworkers to help each other out when possible, but at the same time you are not forced to help them each and every time.

Is there a way to politely decline these requests without seeming like the troll under the bridge protecting his "magic"?

You can always excuse yourself by explaining your coworkers that you have other tasks and projects to do, and that even though you enjoy teaching them doing so too often will surely affect your performance, probably falling behind other tasks (the ones you were actually hired to do).

Is this an indication that I need to either seek a promotion due to these skills, or should look elsewhere?

I see two cases here:

1. If they want you to teach them on a regular basis, additional to the tasks you currently have, then some sort of promotion or raise should be included to compensate for that increased workload you have now.
2. If now some your tasks are to be reassigned so you have the same workload while teaching your coworkers, then a raise or promotion would not be too recommended for this case, as you were "just" reassigned to new tasks.

If you should look elsewhere for a job is really up to you. I suggest you ponder this two cases so you can decide better. Also, I think that speaking with your manager about this situation (and the 2 cases exposed) is also worth a shot, so you can come up with a solution that works for both.

If you find that teaching is something you are not going to enjoy doing then you would probably be better seeking other job, where you do tasks you actually want to do.

However, if you can tolerate, or even enjoy, teaching then there is no need for you to quit. Again, be sure you are ok with the new agreement you come up regarding these new tasks. If you feel you are not being paid what you want for these new responsibilities (even though you enjoy them), then again you would probably do better seeking for a new job. Hope this words help you, good luck.