How old was Jonathan when he met David?

We are not told the age of Jonathan, but we can figure it out approximately. Here are the relevant facts:

1. David reigned in Hebron for 7.5 years (2 Sam 5:4-5), beginning from the time Saul and Jonathan died.
2. Saul’s youngest son, Ishbosheth, reigned for the last two of those years, and died at age 42 (2 Samuel 2:10).
3. Therefore, Ishbosheth was 35 at the death of his father, Saul.
4. 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.
5. 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.
6. 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.)
7. 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.