Being refused for age difference is rare, thou I doubt it would be the only reason someone would be refused for, maybe a secondary reason, and in these times of being PC they wouldn't put age difference as a reason, but more likely that they don't believe the relationship is genuine, due to the fact that any judge at an appeal would over turn the refusal if age difference is mentioned

*the ECO stated the considerable age gap of me and my fiancee.
http://filipinaroses.com/showthread....sal-of-my-visa

Applicants/Sponsors with a large difference in ages The rules do not state that the applicant and sponsor must be within a certain age differential, but if there is a disproportionate age difference (for example, more than 25 - 30 years), additional care needs to be taken. If the sponsor (applicant) has grandchildren that are older than the applicant (sponsor), the application can be tenably refused on credibility grounds. In these cases, professional advice should be sought prior to submitting the application.
http://www.londonelegance.com/transp...s/spousal-visa

Thou, like I said, it's pretty rare to be refused for age difference.