similar case - interview time officer said not needed
some good news for you. I fall into similar category as yours with respect to age. I had that letter requested in the interview notice. I requested ssa for status letter, and never received the letter till the interview date. In the interview, I told the officer that I have not yet got the letter from ssa, and i was over 26 when I entered US. Officer was nice (Houston INS) and said dont worry about that and checked the green card date. She did not even bother to see the passport F-1 visa date.
I will also post my Houston interview experience in another posting. Hope this helps.