Kollel Oholei Yom Tov relates to TLS that boys who were born on the First of Kislev thirteen years ago, (December 2, 2005) will not become Bar Mitzvah this year on the First of Kislev, but rather one day before, the first day of Rosh Chodesh Kislev, the thirtieth day of Cheshvan, (Thursday, November 8, 2018). (Mishna Berura Siman 55 S’if Katan 45.)
The reason for that is, that this year, as opposed to thirteen years ago, since there is a thirtieth day to the month of Cheshvan, it has the halachic status of Rosh Chodesh Kislev, and for all practical purposes, the boy is Bar Mitzvah on the thirtieth day of Cheshvan and is required to put on Tefillin. The issue is further discussed in Shaarei Teshuva ibid. and one should consult with their local Rov.
“People who are not aware of this may inadvertently miss their first day of Teffilin and other Mitzvos,” the Kollel said.