Your question is more about history and politics than about language or pronunciation within Judaism, but to answer requires some explanation of groups and history. To reply briefly is to do damage to the answer, but here goes:
Within Judaism today, there is a division among Jews, depending in part on where their ancestors’ travels took them.
Broadly speaking, those Jews who remained in the areas conquered and controlled by Muslims in the period from 600 to 1400 C.E., including parts of the middle-, near-, and far-east, segments of Africa, part of Europe, and in particular those Jews who had to flee from the effects of the Inquisition (an institution of the Catholic Church, which began in Spain in 1492, and spread to Portugal and the ‘new world’), make up the Sephardi (Sephardic) Jewish population.
Those who lived under Christian controlled rule, who had established roots in Europe, and particularly what was once called Gall (parts of France, Germany, Italy, Spain), make up the Ashkenazi (Ashkenazic) Jewish population.
[The picture is far more complex than this; there were other groups, at various times and places, but this is enough to respond to your question.]
History has led to the situation that the two groups exist, but not in anything like equal numbers. The Ashkenazi Jews far outnumber the Sephardi Jews today (due to various pogroms, war, population growth, the Shoah/Holocaust, and other factors. These two groups developed slightly different dialects of Hebrew over time, with slightly different pronunciation. The Sephardic style was to soften some of the differences in letter and vowels, so that some Hebrew letters, such as the ‘tav’ and ‘sav’ grew to be pronounced the same way, as did some of the vowel sounds.
After the founding of the State of Israel in 1948, which began with many more of the Ashkenazi Jews, many of the Sephardi Jews were forced to flee with only what they had on their backs out of the Arab and Muslim controlled lands, and wound up as refugees in Israel. They were absorbed by the State of Israel, and became citizens. The number of these persons was similar to (if not actually significantly greater than the number of Arab/Muslim persons who fled the area of Israel at the urging of the Mufti of Jerusalem and others who intended to destroy the fledgling state).
By the way, those Arabs and Muslims who fled from the land of Israel were generally not permitted to integrate into the surrounding nations; they were placed in refugee camps, where many of them remain today; they are now called the Palestinians.
As a way of integrating and welcoming the refugees from the Muslim lands, the State of Israel decided to institute the practice of teaching Sephardi pronunciation as the official Hebrew spoken in Israel. Most Hebrew speakers today use this pronunciation. There is a still-sizeable number of Ashkenazi Jews who have chosen to remain with that pronunciation; in particular, the Orthodox (and as some would call them, the ultra-Orthodox) have chosen to hold to the Ashkenazi pronunciation.
So those who speak in the Sephardi pronunciation use the ‘t’ sound, while those who speak in the Ashkenazi pronunciation use the ‘s’ sound. Most of those Jews in America who are not Orthodox, and who were born after the State of Israel was founded, tend to use the Sephardi pronunciation, but many older Jews were taught the Ashkenazi pronunciation. That explains the difference by age.
I recommend that you do some research to fill in the many details that are necessarily left out of this answer.
Rabbi Joe Blair
[Administrator’s Note: Because this is not a question about values, ethics, morals, or right behavior, this will be the only answer provided to this question; it will not have three answers appearing.]
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