Generally I agree with your quote though genetic risk factors are also important. Differences in about 20 genes (probably there are many more) have been shown to be risk factors though none of these have yet led to new therapeutic insights.

With age there is a progressive accumulation of misfolded proteins (notably Abeta and Tau) in the brain. There is good evidence that these misfolded proteins are causal of AD, though how is still poorly understood.

There is a dose-response relationship here - a 40 y.o. will have relatively low levels of these misfolded proteins and so is unlikely to present with symptoms of AD but an 80 y.o. will have relatively high levels of them and so is far more likely to show signs of AD.