I'm not sure what the claim is here.

Kids need diapers for a few years, which we can basically assume is constant. Meanwhile, lifespans keep increasing (especially in the past average-lifespan, say 80 years), and I assume adult diapers are used for significantly longer per person.

At even a 1:1 birth/death rate, given a long enough average lifespan, I would expect more adult diaper sales unless we find better / permanent fixes for whatever causes the need for adult diapers in the first place.

Not all older adults use diapers. In fact, a quite small fraction of them do.

Baby diapers oversell adult diapers in all but extremely freaky situations...

And they have sub-replacement fertility rates -- which means that there are more deaths than births in the long run.