chuonglinh

You're comparing 2 very different departments. There's no practical basis for AOS (in the US) to be faster than consular processing (abroad).
AOS is handled through USCIS (DHS). Immigrant visas are handled through NVC then the embassy, which are both under DOS.
Even then, the specific local USCIS field office (based on where you live) is the biggest point of delays for AOS, whereas the consulate can be the biggest delay for consular processing.

The process is fundamentally the same, except with showing evidence of the father-child relationship instead of bona fide marriage evidence (typically via a birth certificate, although sometimes via a DNA test if there's a potential question of paternity...they would ask for this is necessary).
I believe an interview waiver is available - at the IO's discretion - for a minor child. It's been too long since I looked up the exact rules for interview waivers.

So i guess your PD is 05/04/2016. Final action date for March bulletin maybe at 03/31/2016. Hopefully, your visa will be available before 07/01/2018 where you are protected by CSPA. I believe they will stop counting your age at the time your visa is available. 

So i guess your PD is 05/04/2016. Final action date for March bulletin maybe at 03/31/2016. Hopefully, your visa will be available before 07/01/2018 where you are protected by CSPA. I believe they will stop counting your age at the time your visa is available. 

IF you haven't moved , it will be the same field office you interviewed at to get your GC. You can also mail a singed letter asking to withdraw to which ever center receives all N-400's. You "could technically" still attend the interview and maybe not even get any questions asked BUT you run the chance of  course. I personally became a citizen 2 years ago under the 3 year rule and during my interview NOTHING was asked about my relationship with my wife ect. BUT ive had friends doing the same but THEY have been asked questions about USC spouse and for additional proof that marriage is still good ect.

The visa bulletin is out, as expected 01 March 2016 for F2A.
 
BUT THERE ARE IMPORTANT PREDICTIONS INCLUDED FOR THE NEXT MONTHS:
 
VISA AVAILABILITY IN THE COMING MONTHS
FAMILY-sponsored categories (potential monthly movement)
Worldwide dates:
F1: Up to one month
F2A: Three to five weeks
F2B: Three to six weeks
F3: Up to five weeks
F4: Up to three weeks

I think you will be fine. At least you still have few extra months (time took on I130 approval) from your 21st birthday. CSPA will protect you from that.

Visa Bulletin for January 2018 has come out. Huge change from December from 22DEC15 to 01FEB16. Wish luck to everyone. My PD is 04/11/2016
 

I believe NVC will send out interview letter 3-4 weeks in advance of the visa be available. Your probably received your interview letter in Sept because they knew your visa would be available in Oct. My guessing, They are not working on your case until Oct that's where they figured out that you aged out (again this is my guess). Again its not hurt to attend the interview. Please keep us update  and good luck!