Of course it is, I don't think anyone with half a brain would argue that
That's why if you are going to compete in age classes, then you are best off either looking for a horse born in April or before, or breeding so that they are born before then. If you are not going to compete in age classes then it doesn't matter a jot!
Its totally pointless!! Beau was an October foal, and so barely 10 for 2 months before he is now 11!! I am still going to call him a 10 yr old though!! Well except ofr insurance sake etc
I wouldn't say it was pointless....but it is irrelevant to most people. It is just another way of categorising things for the purposes of eligibility.
Your horse is as old as it is at the end of the day, and is only classified as 5,6,7 or whatever on a certain date for the purposes of being able to compete in specific age classes