Any english equivalent for "前輩" or "晚輩"?
I believe there is no English equivalent, but there are some equivalents that I know of.

Senior/Junior


older/younger in age or higher/lower in rank.


Examples:


  Bob is the senior developer in our team.


Bob 是我們團隊裡最資深的程序員。


  James is 5 years my junior. 


James 比我小五歲。


  Being a senior, I shall take the duty.


作為最資深的人，我應該要拿下這份職務。

Predecessor


person who has a job/position prior to another.


Examples:


  Larry, the new coach of Mickey (I made this up), is way more experienced than his predecessors.


米奇隊的新教練Larry比他的前輩（之前的教練）有經驗多了。



Note the subtle difference between predecessor and senior – predecessor has to be someone that pre-existed in the setting, whereas senior is someone that co-exists in the setting.