I don't believe that simple aging causes pads to lose effectiveness, but other factors associated with years of use can cause them to lose effectiveness:
1. surface may become glazed, or possibly contaminated with foreign materials. Sometimes this can be compensated for by sanding the surface down
a little bit. Put the sandpaper on a FLAT surface and move the pad (not the sandpaper) to remove any surface glase/issue.
i'd only do this if the pads still appear to have alot of life yet and just exhibit a surface problem, otherwise, replace the pads - the newer pads can be had with a
newer sintered metal material that "bites" much better when wet than the old 80's era pads.. I can't remember the brand, but there is one out there that will fit these ATE/Brembos...
2. The pads may be legal thickness, but sufficiently worn so that the piston has to travel much further than when new pads are installed, and sometimes there is
oxidation of an accumulation of stuff (or you may be low on brake fluid) that limits its travel near the "outer limits". More often, the crud accumulation causes the piston
to not retract fully, and you can be left with a dragging brake.. which then wears the pad out faster or glaze it if it gets hot enough.
3. I used to only change brake fluid after it had turned colors, but I am a new convert to the "change it once a year" church. Fresh fluid and fresh brake lines
(brake lines every 5 years or so, depending on type of line and weather conditions) also make a difference.
4. If the rotor is heavily scored, the pads have less surface area to work with, though at some point the pads may develop ridges to match the 'valleys' in the rotor.
In these cases, one really ought to replace both rotor and pads.
5. On one bike I owned, I found that a small, very hard pebble (perhaps it was a diamond ?) had gotten embedded in the surface of the pad. While it hadn't caused
severed scoring of the rotor yet, it WAS causing much of the pad to not contact to rotor surface under all but the most extreme "gorilla-grip" braking conditions.
Removing the pads was necessary to find it - putting new pads solved the problem.