There is very little available data by which to answer this question.
After the death of Saul, David became king of Judah in Hebron (2 Sam 2:11). We are told this happened when David was thirty years old (2 Sam 5:4-5). At the same time Saul’s youngest son, Ishbosheth, began to reign in Mahanaim at the age of the forty (2 Sam 2:10). Saul had three sons, of which Jonathan was the oldest. If Saul had his sons close together then Jonathan may have been forty-two at his death but he may have been older. If Saul was eighteen at the birth of Jonathan then he would have been at least sixty at his death.
This means there is at least a thirty year age gap between David and Saul, and at least a twelve year age gap between David and Jonathan.