Gemara

(Beraisa - R. Akiva): "I will fulfill the number of your days" - these are the years apportioned to a person when he is born. If he merits, they are fulfilled. If not, they are reduced;

Chachamim say, if he merits, they are increased. If not, they are reduced;

Chachamim: It says, "I will add 15 years to your days"!

R. Akiva: The years added were allotted to him from before.

Support (R. Akiva, for himself): It was prophesized "A son will be born to Beis David, his name will be Yoshiyahu". Menasheh (from whom Yoshiyahu descended) was born during Chizkiyahu's 15 'extra' years!

Rejection (Chachamim): The verse says that he will be born to the house of David, but it did not specify that he will descend from Chizkiyahu!

105a - Question: "... That is written in truth" - is there writing (in Shomayim) which is not truthful?!

Answer: "That is written in truth" refers to a decree about which Hash-m swore; it cannot be repealed. A decree He did not swear about can be repealed.

(Rav Shmuel bar Ami): A decree with an oath is not torn up - "Therefore I swore to the house of Eli, their sin will never be atoned with animal or flour offerings".

(Rabah): It will never be atoned with animal or flour offerings, but it may be atoned through Torah.

(Abaye): It will never be atoned with animal or flour offerings, but it may be atoned through bestowing Chesed.

Rabah and Abaye were from the house of Eli. Rabah engaged in Torah and lived 40 years; Abaye engaged in Torah and Chesed, and lived 60 years.

(Beraisa): There was a family in Yerushalayim in which people were dying at the age of 18.

R. Yochanan ben Zakai: Perhaps you are from the house of Eli - "V'Chol Marbis Beischa Yamusu Anashim". Go engage in Torah, and live!

They engaged in Torah and lived - they called the family Yochanan.

Mo'ed Katan 28a (Rava): (Length of) life, children and wealth do not depend on merit, rather, on Mazel;

Rabah and Rav Chisda were both utterly righteous Chachamim. When either prayed for rain, it would come. Rav Chisda lived 92 years, and Rabah only 40.

Shabbos 156a: Shmuel holds that Mazal does not apply to Yisrael:

Some people heading to a swamp passed in front of Shmuel and Avlet (a Nochri astrologer); Avlet said that one of them will not return, he will be bitten by a snake and die.

Shmuel: If he is a Yisrael, he will return (I will pray that he overcome his Mazal).

The man donated his bread in place of someone else (whose turn it was to contribute and did not have) to spare him embarrassment. In his load they found a snake cut in two (he unknowingly killed it while cutting reeds).

(Shmuel): "U'Tzdakah Tatzil mi'Maves" - not only does it save from a harsh death (and cause one to die in a nicer way), it literally adds life!

R. Akiva also holds that Mazal does not apply to Yisrael:

Astrologers told R. Akiva him that the day his daughter gets married, she will be bitten by a snake and die. He was very worried.

On the day of her Chupah an Oni came to the door. Everyone else was busy with the meal, no one heard him. She took her portion and gave it to him. Later, she unknowingly killed a snake.

(R. Akiva): "U'Tzdakah Tatzil mi'Maves" - not only does it save from a harsh death, it literally adds life!

Rosh Hashanah 16b - R. Kruspedai: Three books are opened on Rosh Hashanah: of utter Resha'im, of utter Tzadikim, and Beinonim (intermediate people).

Utter Tzadikim are immediately written and sealed for life. Utter Resha'im are immediately written and sealed for death. Beinonim are suspended until Yom Kipur. If they merit they are written for life; if not, for death.

Bava Kama 50a (R. Chanina): If one says that Hash-m overlooks (transgressions for which there was no Teshuvah, he induces others to sin), his life will be overlooked (he will be prone to calamity) - "Hash-m's actions are pure, all His ways are judgment."

Rishonim

Rambam (Hilchos Teshuvah 3:2): One whose sins outweigh his merits dies immediately due to his evil - "Al Rov Avonecha".

Rebuttal (Ra'avad): The Gemara does not mean that he dies immediately. Rather, it is immediately sealed that he will not live out his allotted years (Yevamos 50a).

Rosh (Teshuvah 53:8): If a woman was widowed two or three times, we say that it is her Mazel to struggle financially, therefore her husbands die. A woman does not go outside; she has no one to support her. We cannot say that it is her Mazel that her husbands will die, for we do not find that this depends on Mazel.

Question: In Shabbos, we say that Ein Mazal l'Yisrael! In Mo'ed Katan, Rava says that some things depend on Mazal (even for Yisrael)!

Answer (Tosfos Shabbos 156a DH Ein): There is Mazal, but it can change through great merits. However, sometimes it does not change. In Yevamos it says that if one merits years are added to him, but if he does not merit years are taken from him.

Maharsha (156b DH Azil): Shmuel did not tell the man not to go because it was a Sefek-Sefeka (two doubts). Perhaps he is a Nochri, and even if he is a Yisrael, perhaps he will overcome his Mazal.

Question: How can R. Akiva say that one cannot merit to live longer? For some Mitzvos (Kivud Av v'Em and Shilu'ach ha'Kan) the reward is "Lema'an Ya'arichun Yamecha"!

Answer (Maharsha Yevamos 49b DH Moshe): These Mitzvos enable one to live the full amount decreed at birth, even if his Mazal was to did sooner.

Acharonim

Yam Shel Shlomo (Bava Kama 5:23): A person should not say that he received more than he deserves.

Question: Life, children and wealth do not depend on merit, rather, on Mazel!

Answer (Yam Shel Shlomo): Ein Mazal l'Yisrael. Hash-m assigns to a person a good Mazal only if he has merits.

Question: If the opinion that Ein Mazal l'Yisrael says that it can change through great merits, R. Chanina must say that even merits cannot change it. The Torah promises rewards in this world for doing the Mitzvos!

Answer (Maharsha Shabbos 156a DH Ein): They argue only about individuals. The rewards are for Kelal Yisrael, all agree that Klal Yisrael is not governed by Mazal.