That is a somewhat iffy conclusion to draw on the available studies, and misses a key point.

There is controversy, and some uncertainty. But the problem isn’t necessarily catastrophic injury during races. The injures that seem to plague training 2 year olds (and they are training when the are younger to race at 2) is in the training more commonly than the races.

The main benefit of racing younger horses is quicker return on investment by the owners.

It has been argued, and this makes sense, that correct and well designed training of horses is more important than the age at which they start. Most current training systems do not provide enough exercise of the right kind in younger horses.

There is research that indicates that two year old racers race longer life-time, and other research that suggests a very high attrition rate of 2 year olds.

Note that this research is all being done in the context of controversy over whether or not to race younger horses, and there is an economic benefit to the owners to race younger horses, and animal cruelty /right/etc activists are likely to skew interpretations the opposite way. So just reading the literature on its own is a bit tricky.

Having said that, here is some of the literature:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4013968/

https://www.sciencedirect.com/science/article/abs/pii/S1558787812000056?casa_token=MkW_WS_r6o0AAAAA:hmuoCrAw0y7LDjGIlZJvncsm_MUvFUkuy0aL5Pdl8_tunwqoYBdXUeePwEvLBPSpp7xbDAGMbzzH

https://www.publish.csiro.au/AN/AN17685

https://pubmed.ncbi.nlm.nih.gov/24954530/

https://pubmed.ncbi.nlm.nih.gov/24954530/

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1751-0813.1999.tb11678.x?casa_token=R-H_xnss8FMAAAAA%3AStkIyYbj7d5CDlcplk5MtkIAg6gQeIYWbAEKtan9r6ytTNL55_ECW3b75v8uw_0KO7pcKHUkcB2kHMSXEg

And for fun, the famous ABC report:

https://www.youtube.com/watch?v=Zp-ALoBRW20

I’ve heard that 2 year old horses get more injuries, but 3 year olds or older are better off.

Go right to the source and ask the horse.

Thoughts on the minimum age required to get that reference?