We are not told the age of Jonathan, but we can figure it out approximately. Here are the relevant facts:
- David reigned in Hebron for 7.5 years (2 Sam 5:4-5), beginning from the time Saul and Jonathan died.
- Saul’s youngest son, Ishbosheth, reigned for the last two of those years, and died at age 42 (2 Samuel 2:10).
- Therefore, Ishbosheth was 35 at the death of his father, Saul.
- Jonathan was Saul’s oldest son, and there were at least three other sons (1 Chr 9:39). So Jonathan must have been several years older than Ishbosheth, and so he was at least 40 at the time of his death.
- David was 30 when he became king in Hebron (2 Sam 5:4) which means there was at least a ten year age difference between Jonathan and David.
- David probably met Jonathan when he entered Saul’s service, and that was probably after the battle with Goliath. (Chapters 16 and 17 of 1 Samuel are usually considered to be out of chronological order.)
- David was considered too young to go into battle at the time he fought Goliath (1 Samuel 17:33) and so was probably no more than 18 years old.
Therefore, Jonathan met David when David was about 18 and Jonathan was at least 28 and probably older.