For those that don't know what Superman and Batman Generations is it's an elseworld storyline where the heroes from their first appearance in the Golden Age would age at a normal Pace, with mantles being passed down. Bruce Wayne passes the Batman mantle on to Dick Grayson with Bruce's son inheriting the robin mantle, this also includes the different members of the JSA who passed their mantles and of the heroes that would form the JLA, such as Jay Garrick passing the flash mantle on to Barry Allen, with his nephew Wally teaming up with Bruce's son, Superman's daughter, and Wonder Woman's daughter.
Young Justice is following a similar pattern where time is progressing normally on Earth 16. For example the at this point in time Young Justice Bruce Wayne is 39 going on 40 by the time Damian is 10 years old and taking up the robin mantle Bruce will be in his late 40s early 50s. At that point it would make the most sense for Bruce Wayne to Advocate and pass the Batman mantle on to Dick Grayson. Which would be very fitting since he would be in his early 30s it would make the most sense that he would be taking on the mentor role for the next generation of the team (ie Robin/Damian Wayne, Superboy/John S. Kent, the Tornado Twins/Don and Dawn Allen, Aqualad/Athur, Speedy/Lian Nguyen-Harper.)