If he and Bidkar rode side by side behind Ahab (2 Kg 9:25)?
Ahab reigned 22 years
So 22+2+12=36 years from the start of Ahab’s reign to the start of Jehu’s reign.
If Jehu started serving Ahab at the beginnig of Ahab’s reign at age 18, that would make him 54 when he became king.
If he started in Ahab’s 20th year at age 18, 34 when became king.