Thanks for the replies. Your definitely correct about him becoming melanistic with age because he looks nothing like he used to. His head is pretty much dark with out any green or red stripes and his shell is also dark.
So he's about 20-25 years old now. So it must take a male res about 6...
I was given this male slider back in 2003. I have had him ever since. That's 14 years. He was and still is about 7 inches long. Out of curiosity, can someone give me an approx age he was when I got him back in 2003. He was fully grown then. Here's a pic I took when I first got him.