Ever since the arrival of the new Batman v Superman trailer, this theory popped up : The Joker used to be Robin. I am here to debunk this theory.
In the Batman v Superman trailer we do see (probably) Joker writing on Robin's suit, which is led by some to believe that Robin actually turned on Batman and became Batman's arch nemesis, the Joker. Now, there are a number of reasons as to why this probably isn't the case, but i will explore one.
The main thought I have towards this theory is that Ben Affleck (Batman) is Forty-Two years old, and Jared Leto (Joker) is 43, now if you made Robin as old as, or older than Batman, that would infuriate comic book fans (as myself).Bruce Wayne is believed to have been Batman for nearly Twenty years (as stated in the BvS Trailer) . So that would make Bruce Wayne become Batman between the ages of 22 and 26. If Bruce Wayne became Batman at let's say 25 years old, then he didn't have a Robin yet. I believe, that Batman got a Robin (who cares which one, let's just say Jason Todd) roughly 5-10 years after becoming Batman (we'll call it 7 years), which would leave Batman 32 and picking up Robin. I can tell that Robin was definitely at least 17 years old by looking at the suit (thank God, a child Robin wouldn't work in this DC universe). Lets say he had Robin for 7 more years before Batman quit; that would leave our Robin at only about 24 years old. Now, what is more likely, Robin dying or Robin magically aging twenty years? You decide.
I know my stance on this seems a little scatter-brained, and theorized quite a bit, but as a comic book nerd, I believe this would be more the route that DC would take.
Batman v Superman: Dawn of Justice hits theaters March 25, 2016.