Aside from a brief (six week) period in April-May, young horses don’t usually race shorter races than older ones.
The reason those early races for 2yos are offered at 4.5f has nothing to do with physical soundness. It’s because first-time racehorses (of any age) need to learn how to concentrate on the job, and giving them a “short” first experience is the best way to ease them into it.
There’s a lot happening on race days: crowds, announcers, other horses and people in the paddock, being ponied for the post parade, etc. Much of this is practiced ahead of time, but it still feels different when it’s the real thing. Which is why very few racehorses of any age begin their careers running long. Even those who are expected to excel at a mile and longer, will usually start with a 5 or 6f race. It’s a mental thing, not a physical one.
Anyone who knows how to train animals knows that several, short, successful lessons are a much better teaching tool than one long, potentially stressful, one.
By June, races for 2yos are offered as the same distances as those for older horses. The short races that are carded are for speed specialists, not (necessarily) for youngsters. It’s about aptitude, not age.