LONDON — The British government said Thursday that it would allow 16- and 17-year-olds to vote, in what it described as a landmark moment for democracy and some of its opponents decried as an attempt to tilt the electoral playing field.

Analysts have described the plan as the country’s largest expansion of voting rights in decades. The last nationwide reduction in voting age, to 18 from 21, came more than 50 years ago.

“Declining trust in our institutions and democracy itself has become critical, but it is the responsibility of government to turn this around and renew our democracy, just as generations have done before us,” the deputy prime minister, Angela Rayner, wrote in an introduction to a policy paper that included the announcement.

The plan also includes promises to tighten laws on foreign donations to political parties and to simplify voter registration.

Here’s a guide to the change and its implications. Several nations do, including Austria, Malta and Brazil, while in Greece the voting age is set at 17.

— The New York Times