Why? Because it works better for the characters they're promoting, maybe.
It ties them better, I guess.
I don't think they're letting Tim being a teen. In fact, I think they'll try to age him a little bit. In that timeline he has been a Robin a whole lot of time, so it will mean that he probably reached 18 when he changed from Robin to Red Robin, from year 4 to year 14.
They're probably aging Jason a little bit too, and making him stay dead for a little more than originally. That depends on how long he has his Red Hood persona before he shows up officially in the timeline as such, in Year 10. That makes 8 years from his death.
I've put all I could learn from those Bleeding Cool readings and my own in this table for Generation 3 here, in case you want to glance throughout it.
https://docs.google.com/spreadsheets...it?usp=sharing
As some other users mentioned, it's not tying 1:1 year in the metaverse (?) with year in reality, and they're probably going to even short it more with whatever happened with the lost time that Manhattan's stolen. But I'm guessing that the gen 3 is pretty safe regarding how much time we see passing in that chart, and how much time actually passed in the comics. My best clue for that is Damian's age, because he seems to age 13 years from gen 3 up to out current point in gen 4. I'm nominating COIE for the event that's going to help then de-aging characters and reassessing time.
They said that what we can see is still in the works, so, pfff. We will see next year, I guess. They could end even dropping the whole concept by then.