The government's policy is that anyone who has under 13 users must be transparent with their parents, and post clear privacy policies. It doesn't prohibit under 13.
It's G+'s policy to prohibit under 13 G+ users, and Google's policy to handle G+ related policy the way they do.
From the linked act:
The primary goal of COPPA and the Rule is to place parents in control over what information is collected from their young children online. The Rule was designed to protect children under age 13 while accounting for the dynamic nature of the Internet. The Rule applies to operators of commercial websites and online services directed to children under 13 that collect, use, or disclose personal information from children, and operators of general audience websites or online services with actual knowledge that they are collecting, using, or disclosing personal information from children under 13.
Operators covered by the Rule must:
1. Post a clear and comprehensive privacy policy on their website describing their information practices for children’s personal information;
2. Provide direct notice to parents and obtain verifiable parental consent, with limited exceptions, before collecting personal information from children;
3. Give parents the choice of consenting to the operator’s collection and internal use of a child’s information, but prohibiting the operator from disclosing that information to third parties;
4. Provide parents access to their child’s personal information to review and/or have the information deleted;
5. Give parents the opportunity to prevent further use or online collection of a child’s personal information;
6. Maintain the confidentiality, security, and integrity of information they collect from children.
Stuff like "obtain verifiable parental consent" is so unfeasible and tangled for any web service with any significant amount of users it is tantamount to prohibiting under 13.
I ran an online web forum in the United States and had the COPPA stuff, had an online fax service where parents could fax signatures. Many kids instead just lied about their age to participate, in the years that I ran the online forum only 2 people ever sent in forms.
Recently I sold the website and the new owner moved it to Europe.
It is a stupid panicy law to begin with, the politicians who came up with it didn't think of the consequences, they just went on with their grandstandings.
As for it's exceptions they are too vague and troublesome to scale for millions of users.
It's G+'s policy to prohibit under 13 G+ users, and Google's policy to handle G+ related policy the way they do.
From the linked act:
The primary goal of COPPA and the Rule is to place parents in control over what information is collected from their young children online. The Rule was designed to protect children under age 13 while accounting for the dynamic nature of the Internet. The Rule applies to operators of commercial websites and online services directed to children under 13 that collect, use, or disclose personal information from children, and operators of general audience websites or online services with actual knowledge that they are collecting, using, or disclosing personal information from children under 13. Operators covered by the Rule must:
1. Post a clear and comprehensive privacy policy on their website describing their information practices for children’s personal information;
2. Provide direct notice to parents and obtain verifiable parental consent, with limited exceptions, before collecting personal information from children;
3. Give parents the choice of consenting to the operator’s collection and internal use of a child’s information, but prohibiting the operator from disclosing that information to third parties;
4. Provide parents access to their child’s personal information to review and/or have the information deleted;
5. Give parents the opportunity to prevent further use or online collection of a child’s personal information;
6. Maintain the confidentiality, security, and integrity of information they collect from children.