First off humans do not stop developing when they are 18. Men really do not stop developing until they are 25 or 27. Women tend to stop developing much earier. I think it is like 19-21.

Horses physically develop much faster than humans. As for as life span is concerned 1 year for a horse is equal to about 4 human years. So a 20 year old horse is an 80 year old person. But it would be hard to measure and compare a horses physical maturity vs a humans. Humans develop over the course of 25 years. Horses develop in a span of 7 years.

But I see no harm in racing horses at a young age. If he we (human) didnt inject drugs and special feed to these horses at a young age then maybe they wouldnt break down so much. If we let them develop naturally then maybe these horses would be sound for a longer period of time.