why do thai men have to be monkhood when they are about 21 years?
The formal name for a monk in Thailand is Pra Piksoo and comes from the Pali language where it is pronounced Bhikkhu. In the present Thai vernacular, most people, however, address a monk as Pra Song. Becoming a monk in Thailand is an intentional act to make merit, especially for one's parents and most particularly for one's mother since she, as a woman, cannot make merit in this way. Although you will see Buddhist nuns in Thailand, called Shee, who have shaved heads and wear white robes, they are lay people who are not fully ordained and who follow eight precepts instead of the 227precepts that an ordained monk must follow.
The decision of a young man to become a monk, and thus to make merit for his family, is an extremely important act. The making of merit should not be looked upon lightly in this context because Thais believe that merit (boon and kwam-dee), plus demerit (bap), is always rising and falling and that one's future, in this life and in the next, can change at any moment. Together, merit and goodness determine a person's level of existence at any moment in time. This is what is generally known as a person's karma. Thais see both their suffering and enjoyment at any time as a result of their own karma, and making merit increases their level of existence in Thai society and hence their enjoyment. By entering the monkhood, a young man can make merit for himself and for his family.