There are so many details I have to ask, is this for a creative writing session, or are you asking based on a real life situation?
That said, your question is oddly phrased as I'm not entirely sure of the relevance of gen z or social media. But at its core it sounds like you are asking are death bed confessions prosecutable?
According to Google yes it's technically possible. But in the scenario you are discussing, where someone is potentially in hospice or in a hospital death bed confessing a crime at an extremely old age, I suspect nothing much would happen.
Often legal issues take a long time for anything to happen, they would have to reopen a case, start an investigation, see if they can find corroborating evidence, ect. Depending on the situation all they would have to go on all they might have is the confession itself and it may be difficult to prove this wasn't some kind of confabulation of dementia.
Assuming they moved forward and the individual made it to trial (which very often can take many years), it's doubtful that you would be able to find a jury willing to convict a 105+ year old person based upon potentially thin evidence.
So yes it's technically possible, but I'm not sure it would be a worthwhile use of judicial resources and suspect it would very likely be swept under the rug.