The first is most common, but is technically incorrect.What is correct?
She can't be much older than me.
She can't be much older than I.
The first is most common, but is technically incorrect.
The second is correct, but sounds old-fashioned, pedantic and forced, to me, at least.
For something that's both correct and normal sounding, use the following:
"She can't be much older than I am."
Than me is only ungrammatical if the author of the grammar disregards the way English speakers use the language. There are a couple of simple ways to make She can't be much older than me grammatically correct. I prefer the simple observation that me is a disjunctive pronoun in this sentence.A perfect example of how language works in real life rather than in textbooks. The grammatical "... than I" sounds unnatural (or pompous) to some people yet the ungrammatical "... than me" is the one used most often, in BrE at least.