Right answer:
Manejé un coche por primera vez cuando tenía 16 años.
To give a response to the question ¿A qué edad manejaste un coche por primera vez? we need to use the preterite tense. This tense is used to describe actions that took place or were completed at a certain point in the past. So manejé is the conjugation of the verb manejar (to drive) that matches the first person singular (yo - I in English) in the preterite tense.