The following are the regulations of Clone Trooper Wiki. Each regulation should be followed by all users. This is Regulation 4 - Clone Trooper Birth Date, explaining why clone troopers can be assumed to have been born c. 32 BBY:
Regulation 4 - Clone Trooper Birth Date
Section 4A, Assumption of Birth Date
All clone troopers can be assumed to have been born circa 32 BBY, unless otherwise specified. This must be placed within infoboxes as "c. 32 BBY".
An exception can be made with clone cadets, as they are younger than regular clones. It is unknown when these cadets were bred, so c. 32 BBY is incorrect.
The following details the reasoning for this date assumption:
In Republic Commando: True Colors, it states that the Null-class ARCs were born around 32 BBY, and the Nulls were the first clones ever produced.
In The Clone Wars: Decide Your Destiny: The Lost Legion, it states that clones are 10 years old from the Clone Wars. That places the birth year at around 32 BBY.
In the Star Wars: The Clone Wars novelization, Rex states (in 22 BBY, 7 weeks after the First Battle of Geonosis) that he is 10 years old. Due to their doubled aging, that places his birth date at 32 BBY.
Before the Invasion of Naboo in 32 BBY, Jedi Master Sifo-Dyas ordered the clone army to be produced for the Republic, as per Star Wars: Darth Plagueis.
Clone troopers were bred twice as fast as regular human beings, so they were ready to fight in the Clone Wars at about the human age of twenty, which meant that ten years would have passed. The Clone Wars started in 22 BBY, so they were born about 32 BBY (see Star Wars Episode II: Attack of the Clones).
On the blog by Leland Y Chee, it states that Boba Fett was born in 31.5 BBY, and he was born at the same time as other clone troopers.
As per the Essential Guide to Warfare, the founding of the Grand Army of the Republic was in 32 BBY, and without clone troopers, there could be no army.
On rebelscum.com, it states clone troopers were bred 10 years earlier. It is most likely referring to 10 years before when they were first seen, in 22 BBY during Star Wars Episode II: Attack of the Clones.
In Star Wars Rebels: Visual Guide: Epic Battles, it states that Rex was born in 32 BBY. His birth would be around the same time as other clones.
Section 4C, Sourcing
This assumption should be properly referenced within infoboxes. This regulation can be used as a reference unless the date of birth is specified elsewhere.
When used in infoboxes, this date can be referenced using this page. The source should read: