Final answer:
Jehoshaphat was approximately 35 years old when he began to reign over Judah.
Step-by-step explanation:
Jehoshaphat was approximately 35 years old when he began to reign over Judah. According to 1 Kings 22:42, he reigned for 25 years in Jerusalem. Jehoshaphat took the throne after his father Asa died, and he was known for his efforts to strengthen Judah's military and religious practices during his reign.