If a prestige class requires 5 skill ranks in a certain skill, is the earliest you can take that class level 5 or level 6
6. You only get 5 ranks in a skill once you take your fifth level in any class and spend its skill points. You can't spend the prestige class' skill points before meeting the requisites to enter it.
I disagree with Lazar. It's a bit of a hazy point tho and might be a house rule but the level you are eligible is when we allow in every game I play.
OK Lazar is on point per RAW:
When adding new levels of an existing class or adding levels of a new class (see Multiclassing, below), make sure to take the following steps in order. First, select your new class level. You must be able to qualify for this level before any of the following adjustments are made. Second, apply any ability score increases due to gaining a level. Third, integrate all of the level's class abilities and then roll for additional hit points. Finally, add new skills and feats. For more information on when you gain new feats and ability score increases, see Table: Character Advancement and Level-Dependent Bonuses.
Kewl thanks