This is true, how we were taught in elementary school is that it isn't grammatically correct if you can't flip flop the subject and object and still have a working sentence. For example:
He is older than I. (Grammatically correct)
He is older than me. (Conversationally correct)
I am older than he. (Grammatically correct)
Me is older than he. (Grammatically & conversationally incorrect, since "me" is not a subject pronoun)
Most people in conversation will use the grammatically incorrect one, and in fact it sounds kind of weird and makes you sound old or too formal if you use the grammatically correct one.
I believe this is what we call as "Purism" which is a sin (well, not really a sin nor a crime, but I like to call it that way) against daily, effective communication/conversation.
Purism is the desire that an idea/item remain true to its essence and free from adulterating or diluting influences (Wikipedia). In conversation, purism applies when one speaks with great reference to grammar rules as stated above.
However, it hinders the real idea behind conversation. As long as the message of the source is completely understood by the receiver, then effective communication already took place.
The above quoted already served as a very good example.
We know through rules that "He is older than I," holds the correct grammatical rules. But, "He is older than me," has been accepted recently because it basically holds the same meaning. Since the meaning of the message has been delivered successfully (we understand its meaning even though it holds incorrect grammar), thus, it can be accepted as an everyday language for conversation.
If you are a purist, though, you should stick with "He is older than I."
Here is another good example that we are very familiar with:
Jean: Good morning! May I please speak with Anna?
Anna's mother: Oh, hello there. Wait a minute, I'll call her.
(After a couple of minutes)
Anna: Hey, Jean.
It's me! What's up?
If you are a rigid follower of grammatical rules (aka conversational purist), you would rather say, "It is I!" But that would totally sound extremely prissy and formal, though. Who on the 20th century would speak "It is I," on the phone? You sound more like a creature-in-hiding and then quickly jumping onto the screen, "It is I!"
The bottom line of this very long post is that, it does not matter whichever you use AS LONG AS your message/idea has been delivered clearly.