The One Where Ross Dates a Student - S6-E18
Ross: I would date her, but there is a big age difference.
Joey: Well think about it - when you're 90...
Ross: I know, she'll be 80 and it won't be such a big difference.
Joey: No, that's not what I was going to say at all. What I was going to say is, when you're 90 you'll still have the memory of what it was like to be with a 20-year-old.
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.