Google is constantly working hard and doing so many changes in all its services including Orkut to improve the things, recently they announced that Orkut birth date would become unchangeable on April 7th, if you have not logged in to your Orkut account recently, please do so asap and cross check your birthday as it will also help in recovering your account in future if you lose access to it.
Might be while using other Google services as well like Youtube and Google Profiles it asked you to confirm your birth date as they are maintaining a centralized database system so that you don't have to enter the same data again and again while using Google services. To check your birth date just visit the link below and make any changes if required
Also inform all your friends to do so even if they have stopped using Orkut because Google is working hard in improving it and I am sure one day they will be back to Orkut (I was wrong here) so better keep the data as accurate as possible in your Orkut Profiles which will be helpful in future and also don't forget to activate the 2-step verification for your Google account which you can see here how to do it so that no one can get access to your account.
via [Orkut Blog]