Since the age varies by region this suggests it is not something controlled by Apple.
For the USA
Legal - iCloud Terms & Conditions - Apple
"I. REQUIREMENTS FOR USE OF THE SERVICE
A. Age. The Service is only available to individuals aged 13 years or older (or equivalent minimum age in the relevant jurisdiction), unless you are under 13 years old and your Apple ID was provided to you as a result of a request by an approved educational institution or established as part of the Family Sharing feature by your parent or guardian. We do not knowingly collect, use or disclose personal information from children under 13, or equivalent minimum age in the relevant jurisdiction, without verifiable parental consent. Parents and guardians should also remind any minors that conversing with strangers on the Internet can be dangerous and take appropriate precautions to protect children, including monitoring their use of the Service.
To use the Service, you cannot be a person barred from receiving the Service under the laws of the United States or other applicable jurisdictions, including the country in which you reside or from where you use the Service. By accepting this Agreement, you represent that you understand and agree to the foregoing."
The above may be a reflection of
https://www.ftc.gov/legal-library/browse/rules/childrens-online-privacy-protection-rule-coppa
Children's Online Privacy Protection Rule ("COPPA")
Children's Online Privacy Protection Act of 1998, 15 U.S.C. 6501–6505
Children's Privacy
"COPPA imposes certain requirements on operators of websites or online services directed to children under 13 years of age, and on operators of other websites or online services that have actual knowledge that they are collecting personal information online from a child under 13 years of age."