The reason these guys tend not to improve much, if at all, from the year they burst on the scene out of nowhere is that it takes years of physical development and training to get to the level where you can travel to run internationally from Kenya, and because getting on the senior circuit is so difficult, it's much easier to lie about your age (most don't get issued birth certificates at hospitals even today in Kenya, let alone 15-25 years ago when these guys were born) and get put on a junior or youth team. So, they're already near their lifetime peaks, or at least only a couple years from them, by the time they compete internationally. Then too if they're willing to lie, they are probably willing to take the drugs the agents feed them or they take drugs in order to get noticed and compete internationally.
As for the birthdate on this guy, it's 2007, so he's supposed to be 16 now, to turn 17 in August, with the 13:05. Totally normal progression. Appear on the scene at 15 with a 3:38 1500m and Commonwealth Games silver in a race with a number of competitors who appear to be their actual age, and then run 13:05 the next year.
History supports everything I have said above. Kenya must have about 400 bans now from 2015 to the present, and we all know how many age cheating scandals they have had.