Last year, Poland's government complied with the Luxembourg-based court's order to suspend the retirements pending a final ruling.
The European Court of Justice should now issue a final ruling finding Poland in breach of European Union law, the ECJ's advocate general Evgeni Tanchev said.
"The Court should rule that the provisions of Polish legislation relating to the lowering of the retirement age for Supreme Court judges are contrary to EU law," he wrote.
"The contested measures violate the principles of irremovability of judges and of judicial independence," he added in his opinion.
The advocate general's opinions are not binding but they are often followed by the court.
The European Commission, the 28-nation EU's executive arm, brought the case of the supreme court justices to the top court in Luxembourg.
The commission has been locked in a bitter battle over sweeping judicial changes introduced by Poland's right-wing Law and Justice (PiS) government since it took office in 2015.
The showdown has led the EU to trigger unprecedented proceedings against Poland over alleged "systemic threats" to the rule of law that could eventually see its EU voting rights suspended.
In early October, the Commission took Poland's government to the ECJ for lowering the age at which Supreme Court judges must retire from 70 to 65.
On October 19, the court ordered Poland to suspend the law "pending the making of an order closing the interim proceedings".
The judges of the top court, who upheld the interim measures in December, are now beginning their deliberations in the case of the retirements.
"Judgment will be given at a later date," the court said.