Qitzur Shulchan Arukh – 181:20
As long as a person remembers, he is able to testify forever, and we do not worry that because the matter became very old, he doesn’t remember it clearly. Even if he only remembers the testimony through writing, that when they appointed him he wrote in his notebook to remember things, and he forgot the matter and only remembers from the writing, he is allowed to testify — but only if when he sees the writing he remembers the event. Similarly, if he remembers the matter by the aegis of someone else reminding him, he may testify, even if the one who reminded him was the second witness. But if the litigant himself reminded him and then he remembers, he should not testify, although the litigant could retell the matter to another part and that party remind him, for then he would have been reminded by someone else.