“Ben-Hur” notwithstanding, slave oarsmen are very rare in antiquity. I can only think of a couple of instances, both during the Punic Wars.
Slaves pressed as oarsmen were normally freed beforehand, though the Romans at least once offered freedom afterwards to those who behaved well.
In boarding actions free oarsmen are additional troops, slaves need free crew diverted to guard them.
Growing old is mandatory, growing up is entirely optional!