Here are my theories as to why older houses are scarier than newer houses:
1) Considering that the house is older, the flooring isn't as stable, so it creaks when you walk. At night, when you walk, the floor creaks, and it makes that creepy sound that makes you believe something is in the house with you... o_O
2) Because you know the house is older, your imagination makes you believe that the house is haunted. Even the slightest knock on the window will cause you to get scared, even though it's just a branch. Because it's old, you get the mindset that the house is haunted.
3) Whether the house is or isn't part of your family, you wonder what kinds of people lived in the house before you. Who knows what kinds of characters have lived there before you. That can potentially scare you, and make the house terrifying. What you don't know scares you the most.
I hope this helped.
Remember, these are my theories. I can not say for sure what makes older houses creepier than newer houses, but I can base these theories off of my own experience with old houses.