In 1 Samuel 13 Saul is 30 years old when he becomes king, so how was Saul's son Jonathan old enough to be in the army and lead a troop against the Philistines?
1. Average life expectancy then was probably around 35-42. I’ve heard of soldiers as young as 15 fighting in the American Civil War.
2. Not quite certain that biblical accounts of age/dates are totally precise.
3. How widespread were calendars? How many people could use them?
The Hebrews were not exact in the way they counted years.
While there are translations that provide an age for Saul in 1 Sam 13:1, the number is missing in the Hebrew text. It has the equivalent of “Saul was ____ years old when he became king.” (As it stands, this line could be read, “Saul was a year old when he became king”—an obvious incongruity!) In many manuscripts of the ancient Greek translation, the Septuagint, 1 Sam 13:1 is missing entirely. Some Greek manuscripts state that Saul was thirty, but this is likely an insertion into those manuscripts on analogy to 2 Sam 5:4, where David is said to have been thirty when he became king. If this explanation is correct, it accounts for the oddity of Saul’s reported age alongside the depiction of his adult child.
It may be helpful to note that 1 Sam 13 does not contain the only oddity in the biblical depiction of characters’ ages. Another example appears in the transition from Genesis 33 to Genesis 34. According to Gen 33:13–14, Jacob’s children are tender aged. In the following chapter (and without any indication of elapsed time in the story), Jacob’s sons are grown and engage in a massacre against Hamor, Shechem, and their town. In this case, the problem is not that the precise age of the characters is missing from the text but that the different accounts come from different authors and were only later combined in the text as we now have it.