They do fade. In regular use( problem 1 - define regular?) Ilford seems to suggest replacement every 5 years or so. I am less sure about fading due solely to age. Deprived of UV i.e. in a box I'd have thought that normal daylight fade would be negligible.

Try each of them on fresh paper with fresh dev and see if each grade gives a noticeably different and progressively more contrasty result.

pentaxuser