Down syndrome-role of mother

[Tyto alba]

Why does increase in age of the mother increases the chance of having an offspring with Down Syndrome?

I want to know the molecular basis of this observation. I'll be grateful if somebody could recommend an authentic source in this regard.

 

[Ygggdrasil]

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3313661/

 

[Tyto alba]

Appreciated.