Nobody says you have to race a horse at 2. No different to drafting footballers at 17. Those that are ready, play in their first season. Others may not play for 2-3 seasons.
Im the mightyquinn raced 8 times as a 2yo. Racing better than ever now as a 9yo.
Our Sixpence raced 7 times as a 2yo. Almost 7 now and had her best ever season.
Forever Gold raced 15 times as a 2yo and now in career best form
Washakie raced 6 times as a 2yo and still racing in GC company at 9
Has the Answers - the ultimate war horse raced 15 times as a 2yo and still very competitive at the FFA level
These are just a few examples I quickly looked up. Im sure there are countless others. The bottom line is that some horses mature early and others later, but just because you race at 2 doesnt mean you wont be racing successfully much later on.