There are many different kinds of dementia... Alzheimer's is only one cause.... dementia itself is really a symptom. A neurologist can perform the necessary tests to determine if what your dad is experiencing is normal age-related memory loss, or if there is something more sinister going on, and if so what is causing it.
My mother (now 83) has Alzheimer's. She was diagnosed at age 70... I would encourage you and your sister to get him to a neurologist for testing and be prepared that he may fight you on it, my mom did. Diagnosis was the scariest time for her as she felt like everyone was working against her. The good news is there are medications now that can slow progression. She takes Aricept which in the beginning actually sharpened her memory a bit. She has still gradually gotten worse over time, but in general I think her progression has been rather slow. I'm fortunate that she still recognizes me even though that's not always the case with other family members.
My father (now 80) has milder memory loss related to a mild stroke he had about a year ago. There are also many other things that can cause memory loss so getting properly tested is important.