My teenage daughter was recently banned from a teen forum. The response we got from the forum administrators explained that she was banned because the IP address revealed that she was "over age" (too old) to join the teen forum. She was using my old computer and I think we all use the same IP address since we live in the same house.
How can they know my age based on my IP address?
Short answer: they can't.
Long answer: An IP address does not carry any information about the ages of any users behind them. The only thing they can do is correlate different activities from that IP address, hoping to find accesses to a service that does some kind of age verification, and then boldly deduce that since one user behind that IP address was verified to be over age, all of the users behind that IP address must be.
An IP address is definitely not the best source of information to match an individual's age.
What I could imagine is that the forum could have an interface to a social network (such as Facebook) and if she has authorized the forum (for example by using Facebook as Iaas [Identity as a Service]) they could retrieve basic information from your daughter such as age, name, city and so on. But never through an IP address.
Why? An IP address, especially for home users is dynamic and can be used by a device that works with NAT, hiding hundreds of users behind it (example an Internet Cafe).
Long story short, the excuse is just not plausible from a technical perspective.
You cannot verify someone's age by their IP address.
What probably happened was your daughter filled out one of those quick age verification checks when she went to the site.
The site admins probably linked her IP to that age verification request.
