The standard in the statute is "reasonably should know" that a user is a minor.
This likely looks something like the status quo, where the site asks for but does not verify your age. Additionally, if the site does not know the user's age but eg categorizes a person as "12-15 years old" for the purpose of ad targeting and sells ads based upon this targeting feature, then courts will likely agree that they should have "reasonably known" that this person is 12-15 years old (else they were committing fraud in selling those ads).
Not much changes in terms of age request/verification. What does change is what you need to do with that info about age once it's furnished and available. KOSA is about TARGETED ADS and ALGORITHMIC RECOMMENDATIONS. It even contains language that absolves platforms of responsibility for content specifically requested by children:
> IMITATION.—Nothing in subsection (a) shall be10 construed to require a covered platform to prevent or pre-11 clude— any minor from deliberately and independ-13 ently searching for, or specifically requesting, con-14 tent; or15 (2) the covered platform or individuals on the16 platform from providing resources for the prevention17 or mitigation of suicidal behaviors, substance use,18 and other harms, including evidence-informed infor-19 mation and clinical resources