Birds are basically today’s lizards and like most lizards can live a very long time and grow throughout their lives. When you think about how big alligators can get in a similar amount of time, it isn’t surprising that chickens can also.
I have a 23-year-old ball python. I can confidently say he has not grown throughout his entire life, and has in fact for the last almost two decades stayed at a comfortable 3.5’ long.