Actually, you do need a comma before the word "but." It indicates that the sentence, "*Your mom's old too," is a complete sentence. The rest is extra information that can be added or not. In this case, "but" joins two complete sentences. Therefore, he can either: put a period after too, capitalize the word "you," drop the comma and the word "and" only put a comma before "but" drop "but" and use a semicolon