How mutations are measured

Comparing the whole genome of a child to both parents identifies new mutations present in the child that were absent in either parent.

An average newborn human inherits roughly 60 to 100 new single-letter mutations from their parents. (https://bionumbers.hms.harvard.edu/bionumber.aspx?s=n&v=2&id=112456)

How deadly are mutations?

Most mutations are slightly damaging, but not enough to kill you quickly. Some are quite deadly and no doubt are associated with the approximate 15% death rate in the womb (miscarriages).

There are two reasons why mutations build up in DNA:
1. Most mutations are only slightly harmful, so the newborn child usually survives and lives to reproduce.
2. But every child he/she reproduces with also has 60+ new mutations.
This means natural selection cannot cannot provide an escape route from a population's mutation buildup. Every act of reproduction is adding 60+ mutations. Adding, adding, adding.
This is observable science.

It is genetic poison to make a habit of mutating protein DNA.
1. A protein must precisely align with its partner proteins and both chemically and geometrically in 3D space (like the 50+ proteins in a ribosome). Random changes destroy its function.
2. Changing the DNA that controls where, when, and how much of a protein is produced can be disastrous. (certain cancers, severe congenital disorders, etc.) This DNA was formerly assumed to be "Junk".

Apart from God's intervention, the human genome will get worse and worse until humans die out.

All life is deteriorating
Mutation buildup applies to all species, not just humans. This is evidence that all life was created, and has been deteriorating ever since. Evolution is a farce.

Due to mutations accumulating in humans, lifespans are deteriorating. The Online Mendelian Inheritance in Man (OMIM.org) catalog tracks over 7,000 phenotype/disorder entries, such as: Sickle Cell anemia, Down Syndrome, etc. New entries are added every month. Improved medical care and better food supply in the last 100 years have temporarily extended lifespans in the U.S. But the unending tide of mutations will win in the end. There is only so much the docs can do.

Historical record of men's lifespans
The oldest record we have of men's lifespans is in the first book of the Bible - Genesis. It records exactly what modern genetics predicts. Lifespans were much longer in early men and have declined over time as mutations build up in our DNA. The earliest men God created lived to be 900+ years old. Subsequent generations reveal a steady decline to the point where we are now - men now live roughly 70 years. The Bible itself records a typical 70 year lifespan for generations that lived many years after the first men.
Bible: Psalm 90:10 The days of our lives are seventy years; And if by reason of strength they are eighty years, Yet their boast is only labor and sorrow; For it is soon cut off, and we fly away. (NKJV)

Note 1: Secular geneticists agree that mutations are accumulating in humans.
Note 2: About 15% of human pregnancies end in a miscarriage. Most miscarriages are due to mutations.
Note 3: All cancer is caused by mutations. Mutations are the leading cause of death in the United States when we combine deaths caused by cancer and miscarriages.