It's level 50 and when you get the add-on The Secert Armory of General Knoxx it turns into 61.
No offense, but ^ is wrong. It's actually 69. My friend has a level 69 Soldier, and I have a level 65 Berzerker.
EDIT: If you do all the patches, it's Level 69. I have a Level 69 soldier as well.