They all go in peaks and troughs.
Puberty plays a huge part in different ways for the girls and boys.
The girls who hit puberty at 9/10 and start periods earlier are over the affects on their body by 12/13 and mikes ahead. They slow down their pbs at 14 and make smaller progress. Those who are later come through then.
With boys those who hit puberty at 13 are swimming massive pbs and quick times at that age and those who don't get it until 15/16 or later come through later. I'm sure you've seen boys stand on a block at 13 with the 16/17 yo and look massive - my ds still looked about 10 at that age 🤣
I've also seen some of those swimmers hit great times at 13/14/15 and win national medals and yet never be able to hit those times again. These are the ones who drop out at 16-18 IME. I don't know if coaches can do anything about that but if all the boys who never came back after covid or have dropped out when starting college they are the ones who couldn't hit times post 14yo they'd hit at that age. They've started getting beaten by kids ranked much lower then them who never even made a counties.
There was that stat I shared from SE where they said that of the top 100 ranked swimmers at 12 (ish) only 11% are too 100 by open age group.
All swimmers can do is focus on their outcome goals and their process to get there. Work on those gains and how to achieve them. Boys especially will be advantaged by the testosterone when it hits but they can't control when this happens.
My da did his biggest gains last year at 17 yo. Quite late for a male swimmer but he's now faster than swimmers he was 5+m behind when they were 14yo.
I've always told ds "no one can swim a 0 time. Everyone has to stop getting faster at some point and you've got to decide how much you do and what you focus on to get the best time for you"
I often get stupid ill informed comments from other parents like "it's alright for you because ds is still making gains".
I remind them that a) it's not about me and it's ds putting in the effort and b) ds spent many a year not moving up squads with his peers and being behind and cheering them on a counties, regionals and nationals etc from the sidelines.
I really do think coming up behind and having to work for every gain has given ds a resilience and work ethic that he's now reaping the benefits of.
Sorry for essay but being a swim parent with a swimmer who always developed slower has given me a real insight into the advantages of this - ones I didn't see and didn't know about when he was 12/13 and nowhere to be seen!