Both different from and different than are accepted in standard American English, and both have been in use for the last 300 years. But is one of these phrases more correct than the other?
In formal writing, different from is generally preferred to different than. This preference has to do, in part, with the historical use of the word than. This term entered English as a conjunction often used with comparative adjectives, such as better, taller, shorter, warmer, lesser, and more, to introduce the second element in a comparison. Different is not a comparative adjective. Thus, when different than first started appearing in English, it sounded grating or less natural to discerning ears.
From has been used with the verb differ since at least the 1500s, which paved the way for different from to be readily accepted into the lexicon. William Shakespeare used different from in The Comedy of Errors: “This week he hath been heavy, sour, sad, / And much different from the man he was…” Other pairings have popped up over the years, including different against, but different from and different than remain the two most useful among English speakers. Different than is common in American English, but might sound strange to British ears, and in the UK, different to is a common alternative that is seldom used in the US.
When in doubt, stick with different from. However, note that there is a time and place for different than. When what follows is a clause, than can be the more elegant choice: My grandmother looks different than I remember. From works best when what follows is a noun or noun phrase: My grandmother looks different from that old photograph of her.