Level 5 - maximum skill is 75 (Apprentice - need to be at least 50 to raise).
Level 10 - maximum skill is 150 (Journeyman - need to be at least 125 to raise).
Level 20 - maximum skill is 225 (Expert - need to be at least 200 to raise).
Level 35 - maximum skill is 300 (Artisan - need to be at least 275 to raise).
Level 50* - maximum skill is 375 (Master) - Primary professions can only train in the Outlands so the account must be Burning Crusade enabled.
* Level 45 minimum required for Fishing, and level 35 required for Cooking/First Aid.
Taken from the trade skills FAQ sticky at the top of the page