The general rule is that the word different (which is an adjective) should be followed by from, though in common usage than is often used. The “purists” apparently argue that the rule is unbending, and the use of than is always incorrect.
The use of from is not difficult in the sentence, John is different from me. From is a preposition and it flows easily. But what of Mary is different than she was a decade ago? To substitute from for than in that sentence makes no sense unless the sentence is restructured; the purists would insist on the revision, but the broader view seems to be that the use of than in that sentence is acceptable because it is not being used as a preposition.
Than in the latter example is correct because it is used as a conjunction between different and the clause that follows. From, which as a preposition must have an object, cannot have a clause as its object. Our suggestion is that you use from in most cases, but than when the use of from would be awkward or makes no sense and different is followed by a clause. (And, of course, we have just lost our purist credentials!)
The rule softens when the adverbial form differently is used. Compare I play tennis differently than he (plays) with I play tennis differently from him; both mean the same thing and both are correct so long as you note the difference in construction; differently is followed by a dependent clause in the first sentence and by a propositional phrase in the second. Even though the word plays may not be expressed, it is implied in the first sentence, and he (not him) must follow than as the subject of the clause. What is incorrect is to say I play differently than him because than is a conjunction and cannot be used as a preposition.
— Ken Butera