Question

why there is errors during dividing of sex cells (meiosis) of the women in advanced maternal age

Answer 1

In Human each cell normally contains 23 pairs of chromosomes, for a total of 46. Out of 46 chromosomes ,Twenty-two of these pairs, called autosomes are same in both males and females.
The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome( 44 + XX)
Males have one X and one Y chromosome.( 44+ XY).
Sperms and eggs are sex cells .

Meiosis is the process in which sex cells divide and create new sex cells with half the number of chromosomes. Normally, meiosis give 23 chromosomes to a pregnancy. When a sperm fertilizes an egg, the union leads to a baby with 46 chromosomes.( Either 44+XX -->Female OR 44+ XY--->Male )

But if meiosis doesn’t happen normally, a baby may have an extra chromosome (trisomy), or have a missing chromosome (monosomy).
Example

A woman age 35 years or older is at higher risk of having a baby with a chromosomal abnormality. As age advances then the errors in meiosis are more likely to occur . This is because the Ovaries have roughly 2 million primordial follicles at birth each containing primary oocyt. The eggs (Primordial follicles ) begin to mature during puberty. If a woman is 35 years old, the eggs in the ovaries are also 35 years old. So chances of meiosis occuring abnormally is more in older females as old eggs are there .
Eg If nondisjunction of chromosomes occur then Trisomy 21 ( Down syndrome) can occur in advanced maternal age .