Democratic Rep. Henry Waxman, a high-profile lawmaker and longtime advocate for clean air and health care initiatives, announced Thursday he won't seek a 21st term in office.

The Californian, who was first elected to Congress in 1974, said he wasn't leaving out of frustration but rather because "it's time for someone else to have the chance to make his or her mark."

"I still feel youthful and energetic, but I recognize if I want to experience a life outside of Congress, I need to start soon," said Waxman, 74. "Public office is not the only way to serve, and I want to explore other avenues while I still can."

With his departure, House Democrats are losing one of their most vocal members on climate change.

Waxman, the top Democrat on the Energy and Commerce Committee, helped usher a cap-and-trade bill through the lower chamber when he was chairman of that panel in 2009 in a Democrat-controlled House.

The bill, co-authored with then-Rep. Ed Markey, D-Mass., made it through the House. But it proved politically divisive, as it died in the Senate and froze talk of climate legislation in Congress until recently.

Markey, now a senator, praised Waxman's work on climate change.

"Henry Waxman is a legendary leader and a public policy powerhouse who will go down in history as one of the greatest legislators in the history of the United States Congress," Markey said. "When we partnered on the Waxman-Markey climate change bill that passed the House in 2009, I saw what the world saw — a master legislator who is driven by his deep passion for the issues and the people he represents."

Waxman pushed for investigations into the tobacco industry and was instrumental in getting President Obama's health care overhaul completed.

House Minority Leader Nancy Pelosi, D-Calif., praised Waxman for championing initiatives to combat the HIV/AIDS epidemic and supporting AIDS research.

"The list of Henry Waxman’s achievements could go on and on," Pelosi said. "He remains as dedicated to his constituents and to the Congress today as he did during his first term nearly 40 years ago."

Waxman served as chairman of the House Oversight and Government Reform Committee before leading the energy panel in 2009, a position he had to surrender two years later when Republicans took control of the House.

Waxman is the seventh House Democrat to announce plans to retire when their term ends in January 2015. Five other House Democrats are seeking higher office and also will vacate their posts next year. Nineteen House Republicans say they will leave office in January.