It's not uncommon for people to make assumptions based on appearances or age differences, and in this case, it seems like some strangers may be mistaking your father for your grandfather due to the age gap between you and your parents. When people see a significant age difference between a child and their parent, they might assume that the older adult is a grandparent rather than a parent.
These assumptions can be based on cultural norms, societal expectations, or personal experiences that lead people to jump to conclusions. While it can be frustrating or uncomfortable for your dad to be mistaken for your grandfather, it's essential to remember that these strangers are making assumptions based on what they observe without knowing the full context of your family dynamics.
If your dad is bothered by these comments, he could choose to respond politely by correcting the mistake if he feels comfortable doing so, or simply smile and move on without dwelling on it. It might also be helpful for you and your parents to have a discussion about how to handle these situations in a way that makes everyone feel comfortable and supported. Remember that families come in all shapes and sizes, and what matters most is the love and care you share with each other, regardless of age differences.