Depends entirely on how the law is written. It is common in situations like this to provide some sort of grandfather provision, ie a two year grace period or something like that.

For those who already meet the ATP mins, your next PC will probably be your ATP check ride.

For those who might fall through the cracks on age, perhaps your employer will freeze your seniority and then reemploy you when you turn 23 and have an ATP. But I suspect that the grace period will allow most FO's to reach their 23rd birthday. If not, you will need a college degree anyway...this would be a great time to go get one.