I have an old house (100 yrs) up off the ground and in the past year the wood floors have began to sag where the boards meet
Most houses built 100 years ago use wooden posts or hollow iron pipes to carry the weight of the main support beam. Over time the columns rust or rot usually at the bottom. Many were not set on footings, but on a large flat rock that was later buried in dirt. Inspect the posts for evidence of rust. Tapping lightly with a screwdriver may produce a difference in tone where the post is thinner. Poking wooden posts with a screwdriver may show evidence of dryrot or insect damage. Any posts that are damaged must be replaced.