Proverbs 3:1-4

1 My son, forget not my law; but let thine heart keep my commandments: 2 For length of days, and long life, and peace, shall they add to thee. 3 Let not mercy and truth forsake thee: bind them about thy neck; write them upon the table of thine heart: 4 So shalt thou find favour and good understanding in the sight of God and man

What a delightful promise of God to His covenantal sons– the promise of the predictable, measurable blessings of longer life expectancy and all around prosperity for obedience to God’s commandments.  These blessings are not the result of striving to earn God’s favor. Legalists can find no solace in this promise. Nor is this the mechanistic workings of natural laws or the quid pro quo of Karma or merely spiritual blessings we experience in our hearts or one day in heaven.  Rather, these are promises for his children today–  a visible display of the favor that God has for all those whom he loves.

The Promise of Long Life (v.2)

Jesus taught us that if we worry and fret we cannot lengthen our life span (Luke 12:25), but if we seek first the Kingdom of God and its righteousness that we can reasonably expect that he would respond with provision and blessing in this life.  One of these blessings are here mentioned by the Psalmist, notably, long-life.
This should be no surprise to us because it is mentioned throughout the Scriptures.  Consider the promises of the Kingdom in Isaiah 65:20 “No more shall there be in it an infant who lives but a few days, or an old man who does not fill out his days, for the young man shall die a hundred years old, and the sinner a hundred years old shall be accursed.”  See also, Deut. 4:40; Jer. 5:26; Eccl. 8:12)
I should also add that this promise extends beyond the life of the believer to his descendants (Psalm 112; Exodus 20).  And because people make up societies we can see that by extension this promise applies to entire societies that embrace His commandments in faith (Deut. 27-28).  Societies under God can expect longer life expectancy and all the attendant blessings of people living longer. I think a survey of christian history and the nations that were more heavily influenced by Christ would reveal this promise to be true. Of course we don’t need stats to believe the promise. But the stats do reveal God to have been faithful. For it is the nations influenced by Christ which by in large have the longest life expectancies.
This of course is not absolutely automatic. Nor is it true that the wicked don’t from time to time see long life.  God is afterall a father and no machine.  But these are covenantal promises through which Jesus rules the world. They are not granted to christians in vain.  These promises are in fact the very future of the world as the nations come to worship the Lord (Ps. 86:9) and the knowledge of the Lord covers the Earth like the water covers the sea (Hab. 2:14).