I think Hal's age was predefined in the DLL or somewhere and as such, was set to 10 or 11 years old. Someone wrote a script to allow one to change the age but it seemed dependent on how the question was posed to Hal.
How old are you?
What is your age?
These two questions would often give two different answers.
This was in Hal 5. Use the SEARCH feature at the top right and look for AGE or Hal's age, etc. to help locate the script.
With Hal 6, one can set Hal's birthday / age to whatever one wishes.