Barny, I may be a bit off in my months but recall that mares come into season in the southern hemisphere in August/Sept & can remain fertile for up to 6 months or so, & their gestation period is 320 days+.

So no, you couldn't have 2 years olds competing against each other with one being 11 months older than the other - I believe the biggest difference would be about 5-6 months