many pre-teens pretend to be older kids and get an account

earlier spring, a friend came to my home. he always plays games with my son. before he left, he told me that my son showed him an online game site which require to be 18 years old, and my son signed in as 21yr. I know he always plays at his friend house on this game site, and the other boy's mom even wanted to celebrate his son's 10 yr birthday with his cyber game friends all over the world. but i never know the age requirment. my son also told me many of his classmates watch the music and video on youtube. i always think he is too young and i don't need to worry about cyber parental control, but i think now it is time to block many sites, even i hate to be police around the house, and really bad to rememer all the passwords for the softwares.