Specific phobia onsets at 9.7 years old on average, and can last 20.1 years on average. So it starts in childhood, and can last a long time. Many people with specific phobia not only have one fear, but have multiple fears (Stinson et al., 2007).
When specific phobia starts in childhood and early adolescent years, the phobia tends to improve and disappear, and within 5 years half disappear (Agras et al., 1972).
So the earlier the onset, the better the prognosis will be.