I don't think its a myth. If you got your lamp and took a photo once a month for a year you would notice that the ends will have a 'blackened' section which increases in length over it's use. It will never go the whle way and meet. lol but it does increase.
Even if the light intensity doesn't decrease this blackening will block more and more light being eminated. Therefore if a new 24" tube emits 18W then an old one with 1" at each end blackened would theoretically be emitting 16.5W with the other 1.5W being blocked.
Not a lot and not proven science but this is what I would assume!!!.
On lights with electronic ballasts this situation takes much longer because there is no flicker turn on like the magnetic ballasts (that need a circular starter) This flickering speeds up the wear of the light.