Why when a women gets older does it become more likely for her child to be born with down syndrome? And how come

Question: Why when a women gets older does it become more likely for her child to be born with down syndrome? And how come chromosome 21 can not be altered or checked in the case of that?
- Keywords:
  chromosome,
  downs syndrome
Asked by anon-244324 to Ondrej, Jordan, Eleanor, Ed on 19 Mar 2020.
- Eleanor Jones answered on 19 Mar 2020:
  
  I believe this is true yes, but I am definitely not an expert on this. A geneticist or a medic would have a much better understanding of this.

Question: Why when a women gets older does it become more likely for her child to be born with down syndrome? And how come chromosome 21 can not be altered or checked in the case of that?