clam,
The starting battery and stereo battery are used very differently so identical batteries will age very differently. Its critical that two paralleled stereo batteries are identical and replaced simultaneously. Its not so critical that the starting and stereo batteries are the same size or age but its not a bad idea for both to be the same type whether flooded or AGM. If my starting battery rests at much less than 12.5 volts after a good charge then I would replace it while upgrading the stereo battery. A dramatic difference in the ability to accept a charge is troublesome.
An old an aged battery is particularly hard on your alternator so there is a time that its more cost effective to replace it even though its getting by.
Some boaters will use two large deep cycles, one for the stereo and one for starting. With a small block in warm weather its not an issue to sacrifice a few cranking amps in favor of the extra amp/hours. This gives you the ability to rotate the battery usage every weekend which extends the life of both batteries. Easily done with a manual battery switch.

David
Earmark Marine