RDB READER: AT WHAT AGE SHOULD I ALLOW MY CHILD HAVE GIRLFRIEND/BOYFRIEND?

This is from a RDB reader:

I'm a married woman with two kids; a boy who is 14 years and a girl who is 11 years. My son recently had a female friend over to the house and when I inquired as to who she was, he told me she was just a classmate. A few weeks later and the same girl visits again, this time they chatted for a longer period than they did the first time. I don't want to ask my son about it again so I don't send a message that I don't trust or believe him. Should I be worried? At what age is it ok for a child to be allowed to have a girlfriend?

Pls share your thoughts and advise this woman. Thanks.