Comment How to guess age without demanding ID (Score 1) 34
It's a hard, if not impossible problem to solve for 100% of people 100% of the time.
On the other hand, if society is willing to live with "you will probably have to show ID if you seem anywhere close to the age limit" then the problem becomes a lot easier.
If the age limit is 12 and you have a 4-digit Slashdot ID, it's pretty safe to say either you are over 12 or the ID wasn't yours when it was created.
Likewise, if your overall "user behavior" is has been consistent with that of someone well over 25 for several years, the odds of you being under 18 are pretty slim.
As a real-world analog, most stores where I live demand ID to buy age-18-restricted products if you LOOK under 30.