I went looking for fics on this a while back, mostly because the idea of the guardians reacting to him being centuries old felt like it'd hit different emotionally. Sandy just sleeping through the centuries is one thing, but Jack actually living through all that time, unseen? That's a whole other level of lonely. I remember one story, title escapes me now, where North finds some record of Jack in a ledger from like the 1700s and just confronts him. The fallout wasn't dramatic yelling, more this quiet, devastating realization from Bunny about all the Easter Sundays Jack spent alone in the snow. It reframed his 'immaturity' as a kind of arrested development, which honestly made a lot of sense.
Another angle I've seen is Tooth discovering it through memories. She's got the baby teeth, so the logic goes she might sense the age gap between his frozen moment and the memories she sees. Those fics tend to get introspective, focusing on her guilt for not knowing sooner. The best ones don't have the guardians suddenly treat him with reverence, though. He's still their Jack, they just understand the weight he's been carrying better.