In addition to other good answers, I'll point out that the distinction of 선배/후배 does not necessarily equal social status (and who gets to say 반말 to who).
For example, let's say 철수 starts his career at a factory and later decides to enter college. Being a freshman, he is introduced to 재인, a sophomore, who is four years younger than 철수.
In this situation, we can say 재인은 철수의 선배입니다.
However, 재인 would normally not say 반말 in the same way he would to other freshmen, because four year's difference is a bit too much. Depending on the situation, they may end up speaking 반말 to each other like friends. Or 재인 might still speak 반말 but do it more "politely": say, "김철수, 한잔 할까?" instead of "야 철수야 한잔 따라 봐라."
Of course, there can be still places where 재인 would do unconditional 반말 just because he's 선배, but these days 재인 could earn a reputation of being a dickhead, or the whole organization could be if such an attitude is widespread.
Everything I said is highly dependent on organizations (and the exact amount of difference in age/seniority), so take it with a grain of salt. :)
If you think "WTF, why is the honorific system so complicated?", yeah, I know. I sincerely hope the whole system burns down within this century, but languages don't change very fast. :/
