It also depends on the type of group it is. I joined my local as a fourth year. In OPA, we've had plenty of pledge classes with no freshmen at all (a trend we made sure to discontinue--at one point, we were about to lose about half of our membership to graduation, so we recruited freshmen girls like crazy that semester!).
Also, my boyfriend joined Phi Kappa Sigma as a fifth year.
Most GT students (I think the number is something like 70%) take more than four years to graduate, and their chapter was small at the time. From my experience, though, I've never seen a fraternity put a cap on how many bids to hand out or pledges to take in. Especially if a chapter is struggling with numbers; they can afford to mostly focus on quantity during rush, then determine quality later.