Would it be impossible to create separation between sites used by older teens and adults?
Obviously it's not impossible, it just requires sites to obtain a verifiable proof of age, i.e., a government ID.
A lot of pathological optimism in this thread, and it might not impact you (at first), but the document you're quoting explains why a lot of people are concerned:
KOSA would require online services to “prevent” a set of harms to minors, which is effectively an instruction to employ broad content filtering to limit minors’ access to certain online content. Content filtering is notoriously imprecise; filtering used by schools and libraries in response to the Children’s Internet Protection Act has curtailed access to critical information such as sex education or resources for LGBTQ+ youth. Online services would face substantial pressure to over-moderate, including from state Attorneys General seeking to make political points about what kind of information is appropriate for young people. At a time when books with LGBTQ+ themes are being banned from school libraries and people providing healthcare to trans children are being falsely accused of “grooming,” KOSA would cut off another vital avenue of access to information for vulnerable youth.
Oops, you're absolutely right about the attribution, the quote I posted above is from an earlier letter, I had too many open at once.
Unfortunately, the provision you mention is essentially a bad-faith attempt to skirt the first amendment objections, while leveraging the imposed 'duty of care' to allow State AGs to censor with impunity. From p.6 of the more recent letter: