USCIS won't deny due to age (unless too young w/o consent). USCIS only checks the eligibility of the petitioner to petition the beneficiary. No relationship evidence is reviewed for the I-129F.
The embassy/consulate will consider all factors to determine if the relationship is bona fide or not. An age gap, if uncommon in the local culture, can raise scrutiny. It is NOT a reason for refusal in itself. Many people with 20-30+ year age gaps have been approved for a K-1 visa. Either the age gap wasn't uncommon, and/or they had enough relationship evidence to overcome a red flag if it was.
Focus on showing how you qualify for the visa, not why they may deny it. Six visits is good, and Brazil isn't "high fraud" for K-1s AFAIK.
Good luck.
