Disclaimer: This is no means by an original idea of mine, but for the love of me, I can't remember where I came across it. I will post an update when I discover its source.
The basic idea for age verification revolves around asking the user an era-related pictorial question. Anybody who's lived through that era should be able to immediately relate to it, while more recently-born users would hardly have a clue.
Here are a few examples that would permit access to users from the 90's, but keep out users from the 2000's:
Q: What's the relation between a cassete and a pencil?
A: To wind up a spool
Q: Identify the brand and type of this music player:
(A: Sony Walkman Cassette Player)
Q: Where have you seen this iconic car?:
(A: Knight Rider)
Q: Identify this popular videogame:
Unless a younger user is quite knowledgeable about older pop culture (or knows about TinEye or Google Reverse Image Search), this age gate mechanism would prove to be quite a deterrent. You would most definitely have a much smaller percentage of break-ins from younger users in comparison to conventional age gates.
On the downside, this approach might be pose a severe implementation challenge:
- You would need a large pictorial question bank, from where your system
would randomly pick questions.
- You would need to implement the mechanism to obtain the answer as a
text box which would mean your must have a tolerance over the answers
('Sony Walkman Cassette Player','Sony Walkman Tape Player','Sony Tape
Player' etc. would all count as valid answers to Q1 above).
- You would need to decide a limit on how many questions a user is allowed to
answer incorrectly before you decide that the user is infact a minor. You might
even choose not to have this limit, and allow the user to try infinitely.
If you're really keen, this is worth a shot!
The questions above are just examples to show how this model could be worked, and do not pertain to actual questions. In order to cater to a larger audience, questions would have to be more diverse. Moreover, users could always retry if they got answers incorrect.
As discussed in the comments below, questions like the "Contra" one pose cultural/localization issues. To circumvent such issues, you'd need to either have a bank of universally applicable questions, or culture-specific questions that can be posed after first asking the user his choice of country/language.
EDIT 2 :
This answer has received enough flak already, and there's no point mulling over it any more. What initially sounded as a fun and creative idea has now dawned on me as being heavily flawed. Let's just shelf this and move on, shall we?