[JR Jansen]

You should be able to dual class at any level, wether that is wise is another thing. For fighters i would recommand to dual at level 9.
The reason why you can't dual class can be one of the following.
1. You are playing a non human char. (not applicable in your case)
2. You are playing a class that can't dual class, like a paladin.
3. You are playing a class that can't dual class to a mage, like ranger.
4. The prime requisites for the dual class aren't met. You should have 15 in the prime requisite for the first class (in your case the fighter class), and iirc, that has to be strength, and 17 in the prime requisite of the second class (in your case mage), wich is intelligence.
5. Their is an alignment requisite (not applicable in your case but mostly forgotten by people who want to dual to a druid)
And you can dual to a conjurer.