Because attraction of water molecules around an ion depends on. that ion's density of charge, smaller ions (and thus ions of greater ionic. potenti. al) attract more water molecules. The result is the. inverse. relationship. between non-hydrated radius and hydrated radius shown below.
[v. 5] In Hebron he reigned over Judah seven years and six months: and in Jerusalem he reigned thirty and three years over all Israel and Judah.   This would have made David 37 when he became king of all Israel. Do not get confused between 'Judah' and 'Israel': they are not the same.
Solomon was prematurely old ( 1Ki_11:4 ), for he was only about 60 at death. However, there is more to this than at first sight. Before King David died in 970BC (Ussher) he told his son Solomon to shew thyself a man (KJV) or be a man (BBE).