In the natural world, fillies stay with their families/herds until they are between 3-4 years old. Then they are chased off by the sire or allowed to be taken by competing stallions. They are not bred as yearlings and two year olds. That only happens in the human world where for some reason people think it is okay to breed animals back to back when they're still technically babies. This is because society is impatient and likes to produce results as fast as possible, which is why we have horses, animals that can live between 30 and 40 years, physically broken down by the time they are four years old.
And no, able to reproduce does not mean sexually mature. Just because they are capable of it does not mean they truly have the physical capacity nor the capability to pull it off in good health. Breed a two year old filly, hope
the baby comes out healthy, and be lucky if she is still rideable or decently useable as a mount in 4 or 5 years when she is so sway backed you can't put a saddle on her because the tendons and fusable bones in her spine haven't grown and strengthened to the point that she can support a growing foal in her belly. Never mind what the weight bearing does to unfused knees and hocks and cannons that are still malleable at that young age.
A horse does not have a completely strengthened and fully supportable bone structure until they are nearly 7 years old. If their backs and their joints cannot take the abuse, why do you think their sexual organs can?
Sorry for the rant.
If you allow a horse to grow, without having to bear serious weight as a young colt or filly, you will hardly ever see swayed backs or serious arthritis in an animal until they're well into their 20's. It's the ones started as two year olds and ridden consistently as soon as they are started that you end up seeing having seriously bad arthritis by the time they are 10 years old. 10! A horse that young shouldn't be arthritic. You shouldn't have to give a cocktail of injections to get a horse in their teens to go without pain. It is friggin' ridiculous because people are so impatient.