There is some black goop called Shoe Goo, made to fill cracks in running shoes. I think it has some cyanoacrylate (Super Glue stuff) built in, which helps it stick in place. I've used it on occasion on tread cracks in tubulars. No problems before the crack and no problems afterwards, and no problems after Shoe Goo. I also used it to glue down a flap of rubber that came up from the casing. A tube of it is less than $20, really cheap compared to a new tubular or a repair at Tire Alert. If it's a YJ tire, the Goo is the price of the tire, and Tire alert is more than the price. But even for a YJ tire, it could give you another 6 months on the tire, and you would have the ability to treat many more tires.

It COULD be just age, but I don't think the light in the photo is good enough to really see much. I definitely have tires >15 years old that do not show spontaneous cracking, and certainly not big cracking. But I really think that if the casing is not greatly distorted when at riding pressure, it has not been damaged and the tread rubber just needs to be adhered to the casing, and the gap filled. The crack could have been started by something on the road like a shard or screw that cut it.

15 to 30 seconds after a tire investment of $50 at the low end not the bottom end? OK, but the Shoe Goo shopping trip will take about 20 minutes to the Local Running Store (LRS) and about 30 seconds to apply it, IF you are careful.