Transitioning is based on social dynamics rather than age. In most cases though, an older fish will have an advantage and be more likely to transition.
The younger, more immature fish will likely have 'more life to left to live' than a terminal male, but there are other factors, such as being lower on the social totem pole, aggression from other fish, not getting as much food. Whereas terminal phase males have strssors, such as defending it's place in the hierarchy and asserting it's dominance.
In some cases females and initial phase males, will continue to transition but not make it all the way to terminal phase in captivity, but that is dependent on social factors, such as if there is another dominant, closely related wrasse.
Paying for a terminal male takes out the waiting and the guesswork. Just beware that many vendors will list a male for sale and say it is terminal phase, but it is just male and not terminal phase male.