I have read -with great interest the various different thoughts about the Bar-Mitzvah date?----Birthdate? Jewish ?/and Secular Calendar.?
As a mother and grandmother- who have had the blessing, honour to celebrate my children/grandchildren's Bar Mitzvah Please allow me to record my thoughts.
Recorded Baby-boy's birth- Secular date 17th August 1948---corresponding to Jewish Calendar: 12th Av 5709 In my view as we belong to the Jewish faith --especially in respect of a Jewish tradition--Bar Mitzvah The Jewish Birthdate is the significant Date of Birth.
Thirteen years later: The Bar Mitzvah date : the 12th Av 5722---corresponds to 25th July 1961
Therefore the now young boy-- who was born on the 12th Av 5709---will be --age 13 --Be Bar Mitzvah on the 12th Av 5722--which happen to be --the 25th July 1961.
There is generally a Seuda-/Dinner -with family friends and Rabbonim --in this case on the 12th Av 5722 The BarMitzvah boy will put on Teffilin--the first time--with his Rabbi and father
On Shabboth--the BarMitzvah Boy is 'called up' the first time as an 'adult' to the Safer Torah and often does the Leyning/reading of 'His Sedra' /the Sedra of the BarMitzvah week
Wishing Everybody lots of Simcha's and Blessed healthy years ahead.