These restrictions are in place to protect children who may have been forced to travel against their will. By forcing children 12 and under to the human facing desks this then means the supervisor can be questioned.
The previous limit of 18 was proving difficult in practice as a number of families were being held up and made to wait.
See the information for if you are under 18 traveling to the UK. For example, a person under 18 traveling alone needs written consent to travel to the UK from their parent or guardian. Checking that would be very difficult to automate.
As well as the restrictions on travelling to the UK alone if you are under 18, where the border guard is probably required to verify that you have parental consent, the automated process requires you to self-certify that you have entered the correct particulars. There is a minimum age below which you will not be considered legally competent to do that, and presumably 18 was chosen as that age.