The new laws force all judges aged 65 or over - including the head of Poland's Supreme Court, Malgorzata Gersdorf - to retire. They are part of a wide-ranging overhaul that the ruling conservative Law and Justice party (PiS) says is needed to rid Poland of its lingering communist legacy.
"I am too old to be a judge," Jagger, 74, said in Polish on Sunday to the audience gathered at Warsaw's national soccer stadium. "But I am young enough to sing."
The European Union, human rights groups, international bodies and Polish opposition parties have all criticized the changes, saying they undermine the independence of the courts and boost the influence of politicians.
Poland overthrew communism in 1989, breaking away from the Soviet system imposed after World War Two, before the Berlin Wall fell several months later.
The key figure in the process was Lech Walesa, the leader of the pro-democracy Solidarity trade union, who was later awarded the Nobel Peace Prize for his struggle.
Before the concert, Walesa wrote to Jagger asking the veteran rock star to intervene while in Poland. On Monday, Walesa thanked the Rolling Stones in a tweet: "True Solidarity will always win."
Poland joined the EU in 2004, becoming the largest ex-communist member of the bloc and a poster-child for peaceful democratic transition in eastern Europe. But the dispute with the EU over the judicial changes and other reforms have tarnished its reputation in Europe.
(Reporting by Gabriela Baczynska; Editing by James Dalgleish and Gareth Jones)