Maybe your co-workers are genuinely concerned about you. Please don't assume that just because they are older than you that they can't relate to what you're feeling.
I think I am probably quite a bit older than you and I can relate to how you feel and what you are doing (with me, it's my right arm). I don't think you should necessarily tell your co-workers everything that is going on with you but you could surely say that you aren't feeling well or that you feel sad.
In the mean time, I wonder if you are getting professional help for your depression? If not, you really should. You don't have to suffer alone.
Don't give up.
