I was looking at the Eldritch Knight prestige class and saw one of its requirements is "Able to cast 3rd-level arcane spells." I thought "Hmm, well my 1st level sorc with 18 Charisma is "able to cast 3rd lvl arcane spells" because his stat is high enough, so if I took Fighter at second level then I should be able to start Eldritch Knight at 3rd." My DM disagrees, reading it as "you have to be a 5th lvl wizard, or 6th lvl sorcerer, etc., to be able to cast 3rd lvl spells."
Most of the prestige classes I see have the same wording, so I figured that was a valid view of it. But then I saw that the Savant prestige class has a slightly different wording for its requirement: "Ability to cast 2nd-level spells."
This made me think they decided to differentiate: to me, while a 1st level wizard might be "able" to cast 2nd level spells because he has 12 int, he doesn't have the "ability" to cast them.
I noticed that most of the time, prestige classes that want to hold level entry at a minimum level will put in direct skill requirements: Dragon Disciple requires 5 Knowledge (Arcana) skill ranks. This means you can't enter it until your 6th level, because you can't have 5 points in a skill until level 5. This is cut and dried with no potential for confusion.
I think that your stat (assuming you have a casting class) being high enough should qualify you as being "able" to cast whatever level spell is specified, though you might not have the "ability" yet. It opens up some more interesting options to casters who might want to get going down a particular road if they don't mind trading in special class abilities for whatever prestige class they're qualifying for.