16 year old house - worsening creaking floorboards.

if the floorboards are nailed and the right nails haven't be used this will happen over time, I would suggest screwing the floorboards as they are and hammer in any protruding nail heads, and be very careful around any water pipes that may be under the boards. I would use a wood screw 45x5

Best option here is to remove a few existing nails to check what length they are & then using screws of the same length carefully place screws alongside existing nails & screw into place this will almost certainly compress the flooring tight to the joists & avoid any pipes & cables

Sounds like the flooring hasn't been glued before being nailed in place. The flooring has dried out and shrounk around the nails so any movement will give you the squicking sound. I would say get hold of some screws and screw the boards down. I would say the screws should be two and a half the thickness of the flooring. So I would be using 21/2" x 10 gage screws. You could get away with 2" x 10 but in a few years could start to squick again. try to find where all pipes and wires run to make sure you don't course problems by hitting them.

1. Disassemble all floorboards. 2. Check the floor joists. 3. Possibly need change or make additional reinforcements for floor joists. 4. Nail back floor boards. (New boards may be needed).