John Gill Commentary 1 Kings 3:14

"And if thou wilt walk in my ways, to keep my statutes and my commandments, as thy father David did walk, then I will lengthen thy days." — 1 Kings 3:14 (ASV)

And if you will walk in my ways
Prescribed and directed to in his word, to keep my statutes and your commandments ;
ceremonial, moral, and judicial: as your father David did walk ;
which Solomon himself had observed, (1 Kings 3:6) ; and whose walk was worthy of his imitation: then I will lengthen your days ;
the other promises of riches and honour are absolute, but this of long life conditional, depending upon his holy walk and conversation; and hence, because he failed in this the Jews observe he did not attain to long life, dying, as they suppose, at fifty two years of age; which is grounded on a wrong hypothesis, that he was but twelve years of age when he he began to reign, and he reigned forty years, as before observed.