It claims to be forced into the action by the Government, but the part of a law it cites is not yet in force.
Faceparty has deleted what it describes as "a huge number of accounts" from its social networking site in recent weeks. It lists 'over 36 years old' as one of its reasons for deletion.
"We understand that only a minority of older users are sex offenders, but you must understand that we cannot tell which," it says in its explanation of the deletion of accounts.
"New government legislation means we need to check older users on the sex offenders list," says its notice. "This legislation is based upon checking email addresses against a government provided list. Faceparty has never insisted on validated email addresses and can therefore not participate in this new scheme."
A new law was passed earlier this month, the Criminal Justice and Immigration Act, which contains provisions for the Secretary of State to require sex offenders to register their email addresses or other information. That, though, is not currently part of the law and would require a ministerial order to become law.
The law makes no reference to the age of offenders. Neither that law nor any other suggests that people over the age of 36 are more likely to be sex offenders.
"Having discussed the use of our website with the home office and the police, and further some pretty serious crimes caused by older users, we were left with no option but to terminate a huge amount of accounts, and without notice, immediately," says the notice.
Faceparty has defended itself against accusations that it is simply trying to create a younger, more valuable user base for itself.
"Despite malicious rumours spread by a few people on the website, it is not true that we have deleted members due to 'ageism'," its notice said.
Faceparty did not respond to a request for comment. It did say in its notice, though, that the site did have a serious problem with sex offenders. It said that accounts were deleted without notice "because a gang of paedophiles had arrived on the website and had carried out a series of attacks on younger users".
The Government has proposed a system which bears some similarity to that described by Faceparty. Home Secretary Jacqui Smith has suggested a future system involving sex offenders registering their email addresses with the Government.
That list would then be made available to social networking sites who could check their users against it in order to help prevent offenders using the networks to contact potential victims.
One part of the new Criminal Justice and Immigration Act allows a Secretary of State to increase the notification requirements of sex offenders. Offenders can already be required to notify details such as their name, address and national insurance number to their local police station.
Those notification requirements could include a person's email address, their passport number, and notice of any foreign travel, the Home Office said in a review of child protection online from 2007.
A spokeswoman for the Child Exploitation Online Protection Centre, a Government-backed centre, said that trying to ban users older than 36 may not be very effective in combating child abuse.
"It's pretty easy to lie about your age," she said. "One of the things we'd like to see all sites do is adopt a Report Abuse mechanism. We think social networking sites are fantastic but providers have a duty of care to children. Children should be able to use these environments but make it easy for them to report abuse."
The Criminal Justice and Immigration Act also made it an offence to possess extreme pornography, a move which had attracted objections from some civil liberties campaigners. That part of the Act is not yet in force.