When Winston goes in to the pub, he wants to talk to the old man and ask him if things were better before the revolution. He talks to the man at length, but can not get an answer that satisfies him.
What he says is that the man is too old to tell him. He says that the many remembers too many random details about his past life, but nothing of importance. He remembers, for example, when he last saw a top hat, but he no longer has the clarity of thought needed to think about a "big picture" question like "was life better before the revolution."
So, the short answer is that the man is too old.
When Wilson enters a tavern to discuss life before the Revolution with an older prole, the man cannot recall whether life was better or worse in the past. Despite remembering useless details from when he was a young man, the old prole cannot distinguish if the quality of life was better before the Revolution. There are several reasons as to why the old man cannot describe to Winston whether life was better or worse in the past. One significant reason that the old man cannot recall what life was like in the past is because the Party has utterly erased history. On an everyday basis, Party members manipulate and erase history to the extent that individuals no longer have a reference point of the past. Another possible reason that the old man cannot recall whether life was better or worse is because he lacks the intelligence to perceive life from a larger perspective. The old prole can only recall insignificant memories. If the old man were to look at history books, he would still be unable to tell what life was like because the Party alters the past.
The man is not too old to recall how Oceania was before the revolution. In fact, he remembers every bit of how life was. The Party, the telescreens, and especially "Big Brother" have instilled fear into him and he knows that he is being watched at all times. Also, the Party can change and erase the history; some of Oceania's history may just be made up lies! The old man knows the real history and by not answering Winston's question, he is able to keep the truth alive...but only he acknowledges what is true and what is not.