Answer
We don't know how old Saul was when he became king. The only verse in the Bible that should have given us this information (1 Sam. 13:1) seems to have been copied incorrectly over the millennia, so that it now presents us with an impossible number. According to the Hebrew text of this verse, Saul was one year old when he became king. But we know from the account of Saul's accession in 1 Samuel 11:15 that he was not an infant. Moreover, if he were one year old, it would be impossible that he commanded an army at the time or within a couple years, and even more impossible that he would have had a son at the time, and yet even more impossible that any such son could also have been a commander in the army (1 Sam. 13:2)! The KJV and NKJV both translate the verse as recording that "Saul reigned one year, and when he had reigned two years..." This translation is problematic in and of itself, not making good sense of the Hebrew.
Some not-so-ancient Greek translations of the Old Testament (Septuagint or LXX) insert Saul's age as being thirty years at this point, but the earlier Septuagint manuscripts do not include the verse at all, thus providing no data. It is hard to imagine that this number could be correct, since Jonathan (Saul's son) commanded one third of Saul's army at the time (1 Sam. 13:2) -- Jonathan would have been incredibly young had Saul really been only thirty (though not nearly as young as if Saul had only been three years old!). In any event, there is no reliable historical basis for that number.
Answer by Ra McLaughlin
Ra McLaughlin is Vice President of Finance and Administration at Third Millennium Ministries.