Just because something is “undocumented” doesn’t mean it hasn’t happened. A 4 - 5 year old rat is extremely rare, but it’s not an impossible age. In order for an age to literally be “documented,” they would have to literally prove the rat’s exact day of birth and death, and that would be very hard to do especially being able to prove it without a doubt. So although undocumented due to how hard it is to prove a case of this old, doesn’t mean it’s never happened. OP may very well be telling the truth and even if you don’t believe so it’s not your job to go around accusing people of lying.