Marilyn318

I'm a Filipina. I lived 30 years in the Philippines and dated many Filipinos before eventually marrying an American.
 
Time for some brutal honesty. 
 
The age gap is definitely a consideration. But for many people, especially those who don't have significant resources of their own, the bigger consideration is money. 
 
I don't know of any 21 year old Filipina with her own resources who would happily marry a 65 year old man. But I can think of many 21 year olds without resources who would do and say anything to land a rich senior citizen. Especially one who offers a path to US citizenship.
 
People will tell you what they think you want to hear because they see you (and your young fiancee, once she gets to the States) as a golden ticket to a better life. They will never admit it to you, of course. 
 
A good test to see if this relationship is genuine: try pretending that you lost all your money (and of course look and act the part). See what your fiancee does. 
 
 
 

My interpretation of the OP is that the K1 applicant is the biological father of the K2.  If that is accurate, no need for adoption papers for one's biological child.  At most, the embassy will ask for DNA test from the father and child.
 

*** Moved from K1 Process & Procedures to the Philippine regional forum, for country-specific input ***
 
 
Your fiance will also need the biological mother's PSA death certificate to show that he now has sole custody of his child born out of wedlock.
 

I'm not sure if it was to prevent child trafficking or because the report of birth was a little delayed from the actual birthdate, but one other thing to consider as happened with our case is at the interview, the Embassy requested a DNA test to establish parent-child relationship.  If this happens, they will tell you the whole process which involves only certain approved genetic labs and test sample collection at the Embassy.  I don't recall that it added much time to the overall process, but it did cause some delay and extra trips to Manila.