At some schools, Rush is almost all freshmen. The houses try to keep the numbers in each class fairly even. Other schools it doesn’t matter at all. The houses also don’t want to have too many in the same class as there could be a huge drop off in numbers when that class leaves. If the house has about 100 members, they want to have about 25 in each class, so if the junior class has only 18 in it the year you are going through recruitment, you have a better chance than if there are 27 juniors. The freshmen classes are almost always bigger than the junior or senior classes, as people transfer, leave school, quit, go on study abroad, graduate early, etc. The houses need those dues, so will take a bigger freshman class, hoping to keep them all 4 years.
So much depends on the school. The big, huge southern schools? Upperclassmen don’t have a good chance of getting a bid if they are 22 years old. At a school that doesn’t have physical houses? Better chance.