Nikon d40 lens fits older?

Older lens fit

Any Nikkor auto-focus and AI lenses will fit, however entry level Nikon cameras like the D40 are only fully functional when using AF-S lenses

AF lenses will work, however you will have to manually focus with them, but the light meter will work.

Other lenses like a Nikkor AI lens, while they will fit, will require you use an external light meter as well as manually focus.

Once you get into the more advanced Nikon bodies, all Nikkor lenses can be used