Aging in this series

Going off a brief Google search, horses can start racing by two years old. For example the Kentucky Derby is run by 3yr+ horses. So it should be feasible that uma musume from S1 would still be active racers even now in S3 if they’re judging ages off of real horses.

I don't undertand too xd, I think that the new girls just grow up fast

Same goes for Teio when she saw Rudolf when she was young. It's just something we have to accept at this point.

Simply don't care that for now
PS: From my perspective if the life of Umamusume is as long as the real horses(20-30yr), it will cause a lot of problems for their average human children: they will probably lost their mom when they are still in kindergarden, but we have seen umamusume mom with their child not only once(RTTT Ep1,mobile game Mr.CB story where trainer meet CB's umamusume mom), so I prefer the conclusion that umamusume can live as long as average human

This doesn't make sense because it doesn't, just bear with it.
Teio and Mcqueen were racehorces from early 90es, rice shower is 2 years younger.
Suzuka and Spe were racehorces from late 90es, slightly before Tm Opera O, TopRo and Admire Vega.
Kitasan, Duramente and Satono Crown were all born in the same year and were racehorces in 2015+.
Unless they would want to fully revamp all cast with every season they are doing it as it is now - mixing up horces from different generations (usual horse racing career lasts for 4 years, from 3 years to 6 years old).