Old parents at a young age

I am writing this to see if anyone else has this same experience or can share some empathetic advice. While my particular situation is not uncommon I still feel so alone and I always fall down the rabbit hole of despair. I am freshly 20 years old and my dad just turned 83 this September, I have struggled with trying to build a relationship with him because of our age indifference. Growing up I used to get made fun of for having older parents. When I think about how much time I have left with him it’s almost like a ticking time bomb, I wonder what life will be like and how I will grieve, it hurts me to think about the times I’ve been rude to him and my frustration towards him, is it selfish of me to think this way? I don’t know if anyone else can relate to my situation but I feel so unfulfilled and I want to find ways to spend the rest of his years building towards a healthier relationship.