Also depends on the age of the building. Some of the ones in our area have no air conditioning at all (yes, seriously--and in a hot, humid place).
The one I lived in has wall units under the window (hitting a few inches under the window sill). Air blows out from the units. Each one is connected to the one below/above it, and a big system in the basement. No one is going to do a Voyage to the Bottom of the Sea to it (a show well-known for people using the air vents to crawl around).
Though, overall, I'd say no, not for an apartment building. The vents would need to be awfully large to accommodate a fully grown person, and I can't see a reason why they would need to be that big (and, besides, they may be intentionally small now so that people can't use it to break in). In all the places I've been--Army WWII barracks, apartments, houses, the vents have always been pretty small. Don't need a lot to get the air out into the place.