1. Tosfos (28a, DH u'Mitah) writes that Kares is from 50 until 60, "v'Lo Shishim bi'Chlal" -- and year 60 is not included.

2. I found a commentary on Moed Katan by Rabeinu Shlomo ben ha'Yasom (as recorded in the introduction there, he is believed to have been a contemporary of Rashi and lived in Italy), who wrote that "Yom Echad b'Shanah Chashuv Shanah" -- one day in a year is considered a whole year (see Rosh Hashanah, beginning of 2b). According to this, when Tosfos cites in the name of the Yerushalmi that no one of the generation of Bnei Yisrael in the Midbar died under the age of 60, this means that they all lived at least 39 years and a day more after reaching the age of 20.

According to this, Kares ends at 59 years and a day.

Kol Tuv,

Dovid Bloom

I found another source which comes to a slightly different conclusion than what we saw in our first rely.

1. The Yam Shel Shlomo (written by the Maharshal) on Yevamos (chapter 6, end of paragraph 8) cites the Yerushalmi (cited also in our first reply) which seems to maintain that dying at the age of 60 is not considered Kares but rather considered as dying at the hands of Heaven, similar to those who died in the desert. The Yam Shel Shlomo writes that it is possible that the Yerushalmi does not disagree with the Bavli that Kares applies between 50 and 60, because one can argue that the Bavli states between 50 and 60 and does not include the 60th year. The Yam Shel Shlomo concludes that we will not be able to decide this issue until Eliyahu ha'Navi comes and enlightens our eyes in the understanding of the Torah.

2. Accordingly, the view of the Yam Shel Shlomo is similar to that of Tosfos in Moed Katan (28a) that we saw in the previous reply (and, in fact, it is intersting to note that the Yam Shel Shlomo does not cite the Tosfos), but he does not want to decide definitively that the 60th year is not included in Kares but instead writes that this is a matter of doubt and we will only know the truth when the Mashi'ach comes.

Dovid Bloom