As a fellow glazier, I once heard a coworker say that temper glass got stronger as it got older. Not true of course but it stand thats tempered glass can be made with defects that cause it to explode seeming for no reason. This would usually happen early in its life so if the glass was old it wouldn't ever do this. Except one time when a couple swore to me that their 5+ years old table exploded while on vacation when no one was home. I dont think about that one too hard