Hi
As noted above, the date codes on the tires do matter. If there was significant sun exposure while sitting a for a few years, that also matters. Tires don't like lots of UV exposure. Also as noted above, the brand / model of tire you have matters. The Marathon's ( and a few others ) don't have a great reputation for longevity.
Is there a magic date? Most get worried at the 5 year point. You look at the tires to see what's going on. Almost inevitably you find this or that. Some of it could date to the day you pulled the trailer out of the dealer lot and bumped that long forgotten curb
. Some things (major cracks) are red flags in terms of replacement. Judging exactly what is "major" is not as easy as it might seem.
Your trailer is worth a lot of money. A pair (or a set of 4 depending on what you actually have) are a lot less expensive that even minor damage to the trailer. Unfortunately the clearances on the AS wheel wells make it possible that a full blowout can do damage. No, it's not likely .... how lucky do you feel?
Our answer on a 2017 was to replace the tires a few months back.
Bob