I think we can reasonably suppose that the quality of the human constitution was very high at the beginning. Long lives would accelerate the populating of the earth in line with God's commandment to be fruitful
The lifetimes given for the patriarchs are as below
Adam - 930 Genesis 5:4
Seth - 912 Genesis 5:8
Enosh - 905 Genesis 5:11
Cainan - 910 Genesis 5:14
Mahalalel - 895 Genesis 5:17
Jared - 962 Genesis 5:20
Enoch - 365 (translated) Genesis 5:23
Methuselah - 969 Genesis 5:27
Lamech - 777 Genesis 5:31
Noah - 950 Genesis 9:29
Shem - 600 Genesis 11:10–11
Arphaxad - 438 Genesis 11:12–13
Shelah - 433 Genesis 11:14–15
Eber - 464 Genesis 11:16–17
Peleg - 239 Genesis 11:18–19
Reu - 239 Genesis 11:20–21
Serug - 230 Genesis 11:22–23
Nahor - 148 Genesis 11:24–25
Terah - 205 Genesis 11:32
Abram (Abraham) 175 Genesis 25:7
Isaac 180 Genesis 35:28–29
Jacob (Israel) 147 Genesis 47:28
Joseph - 110 Genesis 50:26
Moses - 120 Deuteronomy 34:7
Joshua - 110 Joshua 24:29
David - 70 2 Samuel 5:4
There is a progressive decline in lifespan after the flood.
But life expectancy has remained fairly constant since the days of David.
Psalms 90:10 The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away.
Unfortunately, we are not given the reason for this.
When the kingdom of God is established on the earth life expectancy will increase.
Isaiah 65:20 There shall be no more thence an infant of days, nor an old man that hath not filled his days: for the child shall die an hundred years old; but the sinner being an hundred years old shall be accursed.
I hope you find this helpful.