1. The Birth and Death of Typhoons (Tropical Cyclones)
The Definition of Typhoons (Tropical Cyclones) and Its Birth
Typhoon is defined as a tropical cyclone with the maximum wind of 34 knots or higher. A tropical cyclone with the maximum wind of less than 34 knots is called a tropical depression. Difference between two types of tropical cyclones are just if the maximum wind is less than or above the threshold of 34 knots.
The real meteorological phenomena, however, change continuously, so the distinction of two types of tropical cyclones is not clear. Moreover, the ground-truth measurement of the maximum wind speed around the center is not always possible, so it is not possible to decide automatically if a tropical cyclone is a typhoon or not. This decision making requires the skills of human experts and the collection of information.
For example, in Japan, experts in Japan Meteorological Agency estimates the maximum sustained wind around the center based on the comparison of various meteorological data, and they declare the birth of the typhoon when they are sure that the maximum wind exceeds the 34 knots threshold. It does not mean that American experts completely agree with the judgment. They may have different view on meteorological data, and have different judgment on whether the maximum wind is over or under 34 knots. This example indicates that decision making by experts may differ from country to country, so the meteorological organization in each country makes independent judgment on the birth of the typhoon. In the Western North Pacific basin, however, the decision made by Japan Meteorological Agency goes as the internationally official decision.
Definition of Life Period
The involvement of human decision making, in turn, leads to the explicit definition of the birth and the death of the typhoon. For example, we cannot define the birth and the death of tropical cyclones in general, because they are inherently one instance of the continuum of natural phenomena. For typhoons, however, human experts declare their birth and death, so we can explicitly define the life period of typhoons. This is one of the unique characteristics of the typhoon.
There is nevertheless a small issue on how to define the period of the life. This is because some typhoons come to life again after weaken into tropical depressions. We even have a typhoon with two rebirths (Typhoon 197306). Hence there are two possibilities as to the definitions of the life period of the typhoon in those cases. You can see the list of typhoons with different definitions of lifetime in the list of rebirth typhoons.
This site employs the latter definition, which excludes the period of the typhoon's being a tropical depression. This definition sometimes conflicts with the length of life appeared in other materials, but this difference can be understood as the difference of definitions.
2. The Typhoon Is Changed to the Extratropical Cyclone
We often hear in weather forecast that "the typhoon is changed to the extratropical cyclone (or extratropical low)." What does it actually mean?
An expression like "the typhoon is changed to the extratropical cyclone" is in fact used for meaning the change of structure, not the change of intensity. More specifically, this expression is used when the structure of the typhoon is changed from a tropical cyclone to an extratropical cyclone. In most cases this change does not go backward.
On the other hand, a similar expression like "the typhoon is changed to the tropical depression" is in fact used for meaning the change of intensity, not the change of structure. More specifically, this expression is used when the maximum wind around the center happens to be smaller than the threshold to be a typhoon. It only means that the maximum wind is below the threshold, so we sometimes see a tropical depression revives as a typhoon.
In short, change from a typhoon to an extratropical cyclone means structural change, while change from a typhoon to a tropical depression means intensity change. If you know this difference, you can naturally understand that an expression like "the typhoon is weakened to become the tropical depression" is almost correct(Notice 2.1), but an expression like "the typhoon is weakened to become the extratropical cyclone" is wrong. It is not rare to see "the typhoon deepened to become the extratropical cyclone" (eg. Typhoon 200418).
Of course, this change is a continuous process, so we cannot determine the exact timing of the change. Experts declare the change at an appropriate timing when they believe that the features of an extratropical cyclone is dominant.
(Notice 2.1) This expression only applies to the wind, so we should be aware that this expression does not imply for the typhoon to be weakened in terms of rainfall.