AEP, the nation’s biggest coal-based utility, said it would shut down five aging coal plants, convert at least two others to natural gas and retrofit a dozen more as part of a $6 billion to $8 billion plan it said would help it comply with proposed Environmental Protection Agency regulations.

AEP joins other utilities — including the Tennessee Valley Authority, Dominion Resources and TransAlta — that have decided to close coal plants and upgrade others for economic reasons or in conjunction with EPA negotiations.

Coal-fired plants account for nearly 25,000 megawatts, or 65 percent, of AEP’s total generation capacity. AEP’s plan would close down 6,000 megawatts of coal-fired units. The five plants that will close include units dating back to 1944; the newest of those units is 51 years old.

AEP chief executive Michael G. Morris complained in a statement of “unrealistic compliance timelines in the EPA proposals” that he said would force AEP “to prematurely shut down nearly 25 percent of our current coal-fueled generating capacity.” He predicted higher electricity rates.

But environmental groups said that the plants to be closed were too old to be economically viable and that AEP needed to comply with a court order to curtail toxic emissions. Mary Anne Hitt, a Sierra Club official, said those emissions contribute to thousands of premature deaths from asthma and heart attacks and added $62 billion a year to health costs.

“EPA regulations do not require any power plants to shut down,” said Vickie Patton, a lawyer at the Environmental Defense Fund. “Companies like AEP make the decision — either invest in common retrofits like scrubbers to clean up pollution, or close down old and poorly controlled plants and replace them with cleaner, more efficient generation.”