Do you have any concerns about your batteries today? If not, and you are basing this more on the age of the batteries, than I would go through the following steps:
1. Research the battery to confirm their age. Depending on the make, the date of manufacture could be on a sticker either on the side or top of the battery or an engraved code on the cover. For example, Trojan batteries list two codes near the Pos and Neg terminals - the Pos has a manufactured date with a code for month and day and the Neg has a shipping date with a code for a month and year. Other manufacturers do it slightly differently so go to your manufacturer's website for how to read the code.
2. Test your batteries to verify or have a shop do this for you. Many well-maintained deep cycle batteries from a good company can last many years. Its really dependent on how well the batteries were maintained, though so its a good idea to test the batteries. Here is the Trojan manual
that described four methods for testing (P17).
I do not have much experience with AGM so I'll leave that to others, but plenty of people use them and there are positives and negatives associated with each AGM or flooded. Here's a link to Windsun that I think is really good for battery information.
Deep Cycle Battery FAQ