Minimum Level for Prestigue Class

[VegasHoneyBadger]

Dragon Disciple requires 5 ranks in Know Arcane to qualify. Can I take the prestige class at level 5 while taking my 5th rank, or do I need to wait until I am choosing my 6th level class

[Rathendar]

You have to pick the class for level 5 before you know how many skill points you get to assign, so no. you have to wait for DD til 6th.

[Duiker]

6th level. This has been answered in various FAQs. You don't get the 5th level skill rank until AFTER you select a class, and you can't use the class to qualify for itself.