The Building Safer Communities Act = Infringement

The provision is an enhanced review for anyone under 21 allowing NICS to take as much as 18 business days for their approval. I don't know what would trigger NICS to ask for the additional 10 business days, but the law gives them that additional time to complete the background check.

BIPARTISAN SAFER COMMUNITIES ACT
The Bipartisan Safer Communities Act is a commonsense, bipartisan proposal to protect America’s children, keep our schools safe, and reduce the threat of violence across our country.

SUPPORT FOR STATE CRISIS INTERVENTION ORDERS

PROTECTIONS FOR VICTIMS OF DOMESTIC VIOLENCE

CLARIFIED DEFINITION OF 'FEDERALLY LICENSED FIREARMS DEALER'

UNDER 21 ENHANCED REVIEW PROCESS
Requires an investigative period to review juvenile and mental health records, including checks with state databases and local law enforcement, for buyers under 21 years of age.
NICS will have up to three business days to conduct the initial enhanced search. If that search reveals a possible disqualifying record, NICS will have an extended window of no more than ten business days total to complete the investigation.
Provides additional funding to the FBI to administer new process checks in NICS and grants to help states upgrade criminal and mental health records therein.

That's more a question for Legal instead of Activism, but if the FFL and NICS are following the law as written there's no "push back" without deciding to become a test case and file suite against the law and fight through the courts to SCOTUS to have it changed. that isn't as far-fetched as it seems since treating 18-21 year olds differently than 21 year olds is a precarious position for government to take. Contact the 2nd Amendment Foundation and ask if there's an existing lawsuit that you can join for the rights of your 18 year old Eagle Scout being infringed.