No question that there are good and bad ghosts, and I usually like to believe that ghosts don't mean to cause harm to us. But here's my answer to your question.
Ghosts linger in homes and houses mainly because they were killed, so they had to stay behind and linger, waiting to be set free, or to be avenged. They can't rest peacefully until then.
Maybe
There are no such things as ghosts.There are ghosts and ghosts don't enter you... poltergeists POSSESS you. O.o
Ghosts can choose to stay on earth. Some might have some unfinished business or they want to stay with their loved ones. To see ghosts it depends if you have the gift to see them.
Ghosts can be anywhere! Look out, theres one behind you!!! :) JK(Just Kidding :))
they do not age they stay the way they died
They don't. The only place they stay is in your mind. Ghosts do not exist.
to stay behind or to be left
The story behind Gettysburg ghosts is due to it being the largest battle in North American history, with many souls that were never "put to rest" due to the nature of the war.
Ghosts are just demons, I am in a Christian house, so no. Do they live in yours? Try repelling them in the name of Jesus.
Becasue they feel like its there home
Ectoplasm
Obviously. There's one behind you right now.