I too would say it depends on the individual horse. We had a Sept. 2nd filly last year. In late April when she was 7 1/2 months old, yes, she could have been competitive in the yearling classes. She'd have been slight compared to some, so if the judge happened to prefer a more mature type of yearling she wouldn't have done well. If the judge liked 'em leggy & slim, though, she'd have done great. By late May, though, this filly had gone into a growth spurt & was so butt-high & gangly looking there was no way we could show her at that time. Now she's just over a full year old & is starting to look good again. Had she been a February baby, we'd have probably done well showing her this year. She's a tall, leggy B filly, just the sort to be prone to sudden growth spurts & gangly spells.
A foal of different breeding may not have those growth spurts or any gangly stages...we have had some spring foals that looked great at the age of 9 months, and had they been fall babies they would have been fine showing with the full yearlings the next spring/summer.