Two parts of the year and the connection with the life expectancy of a person
In vedic tradition, the year is divided basically into two parts - a period when the days become longer, and the period when the days become shorter.
The first part is called uttarayana (sanskrit: उत्तरायण), second - dakshinayana (sanskrit: दक्षिणायण). These periods are usually observed in either the Northern Hemisphere or the Southern Hemisphere. There are places on Equator, where the whole day of the year is longer than the night, but we will not consider these exceptional cases.
Spring and autumn can be seen in a mirror image of Northern and Southen hemispheres of the Earth.
We currently live in the last of 4 Yugas - called Kali Yuga. Duration of Kali Yuga is said to be 4800 years. Total duration of all Yugas is 12000 years. That means that our civilization lived on this planet from start of first Yuga - Satya Yuga - quite long time.
The duration of human life with each Yuga decreased. It is related to human karma - Kali Yuga carries out the worst karma, but that happens the fastest, so it is enough to have a relatively short life span to fulfill the bad karma of current incarnation. It is said that in Satya Yuga average human lifespan was 100000 years, in Treta Yuga - 10000 years, in Dwapara Yuga - 1000 years, and now in Kali Yuga - around 100 years.
120 years and 108 years
In fact - in Kali Yuga we can provide two life expectancies - 120 years and 108 years. 120 years for those born in spring time (days become longer), 108 years for those born in autumn time (days become shorter).
We must say - this life expectancy was at the beginning of Kali Yuga. We live in the very end of Kali Yuga. This period is called Kali Yuga Sandhi. So we can see that mostly all people die much earlier than they should - and all this is happening with our "super developed" medicine.