Tim had his 16th birthday before OYL. How could he be 15? (I remember him mentioning that having a legal guardian wouldn't matter when he was 18 in one of the last Robin comics, which gave me the impression that that day wasn't far off. Which is why they're aging him up to Red Robin, an adult character, and Dick is saying he's not Robin because now he's his equal, which he would never do if he was 15--that's still Robin-age.