What does the "three-year-old luck" written after Kun Zao mean when doing horoscope calculation?

Good fortune begins at the age of three

A person's fortune generally changes every 10 years

That is to say, good luck begins at the age of three: 13 23 33 43 53 63 ....

The next fortune will start in these years

But it is not so stable. Sometimes it will move forward a little and sometimes it will be pushed back a little