doesn't go by the month, when using the terms like you are asking about. goes by the year.
A young bird is considered so, the year it is born. The next year, it becomes an old bird. Doesn't matter if it hatches in Jan. or Dec. Come Jan 1st of the following year, it's considered and old bird.
Example..........2009 banded birds are young birds right now. In 2010 they will become old birds. In 2020, they'll still just be old birds. Sometimes, a 1 year old is called a yearling too, but they are still an old bird as far as racing them goes.
You can't race a young bird with old birds and vice versa.
PS: It SEEMS like I read something "new" about young and old birds being shipped together, but I'd have to check on that.
Young birds are birds less than a year old. Old birds are yearlings (one year old) and up. Only birds banded in the previous year or older can race in old bird races. Only birds banded in the current year, can race in that year's young bird races.
So by the time young birds start racing, they're usually at least 5 months old.
And in old bird season, it varies.