Optima, IIRC is a gel cell, which is a variant on AGM, but not really an AGM battery.
AGM uses an absorbed glass mat, like a sponge, to hold essentially the exact amount of electrolyte. This is different than a gel cell which uses silica gel to solidify the electolyte for spill reasons. In an AGM, the electrolyte is wicked.
Now, the main thing with these batteries is that as opposed to flooded batteries, even low maintenance ones, there is a valve that prevents gas from escaping so that the H and O can recombine to form water. This prevents dry-out and change in acid concentration to a range that is not optimal for life and performance.
So in theory they can last a lot longer, especially when left on float service in cold temperatures.
Nothing kills lead acid (or any) batteries better than heat. Heat drives side reactions and other degradation. It is well established that a small temperature rise (single digits over room temp) will half the life of a lead acid battery.
The good news? Storing cold slows all this.
The bad side? Cold temperatures drop the cell voltage. Not a big deal if not under load, but a bigger issue if put under load because then the current flow goes up to meet the power demand, etc.
The best thing, as said, is to keep it on a float charger that is thermally compensated, as the required float voltage is temperature dependent. It is also best to have a smart charger that can actually vary current at that voltage very well.