Canon's old lenses need adapters to work on anything new
Nikon's old lenses need adapters to work on new Canon or some other bodies.
Nikon's old lenses still work on new Nikon bodies
without adapters (with some AF issues on low end bodies)
Therefore, Nikon lenses are more versatile, and there's a pile of old used ones to be found out there, more than Canon's.
True.
But.
Not all things are equal.
Some Nikon Lenses are better than similar Canon lenses of that time.
Other Canon Lenses are better than similar Nikon lenses of that time.
Also Nikon shooters like Nikon Lenses and Canon shooters like Canon Lenses.
So blanket statements are broad, comfortable and kind of cover all the bases but are just barely more right than wrong in any given specific case.
Absolutely!
Do not try using a Nikon non-AI lens on a new Nikon Consumer body. Or one of the "G" lenses that were optimized for APS-C before Nikon actually had a FF digital camera.
General statements lead the unaware into actually believing the misinformation and even into damaging their equipment.
You need to carry a large table around with you if you want to use old Nikon lenses on a new body, or at least have a good memory.
Those wanting to mount old Nikon Lenses need to read this first, or risk damaging their digital body!
http://www.nikonians.org/nikon/slr-lens.html
http://www.aiconversions.com/compatibilitytable.htm