Girls: If a 39yr old man that you have had a close relationship with gave you a bday card and he wrote “You are out of this world” in it and signed
“love, _____” how would you interpret this?
Guys: what would be your reason to give a girl a card and write “you are out of this world!!” in it and sign it with love?

I would not think about it enough to interpret it. He thinks you're cool?

Depends on the nature of the close relationship, if it's a romantic dating one, or just deep friendship. Or maybe you dated and remained friends. The devil is in the details.