Who was the oldest player playing international cricket in March 2021, and who was the oldest ever to do so?

The oldest Test Match cricketer still playing in March 2021 was Dilruwan Perera of Sri Lanka, who was born on 22 July 1982. He was eight days older than the next oldest, James Anderson of England.

The oldest player in his final Test Match appearance was Wilfred Rhodes of England, whose last appearance was in the Fourth Test Match at Sabina Park in Kingston against the West Indies, which ended on 12 April 1930, when he was aged 52 years and 165 days. Rhodes played in a total of 58 Test Matches over a thirty year period from June 1899 to April 1930.

The oldest person to make their debut was James Southerton of England who, at the age of 49 years and 119 days, played in what was later classified as the very first Test Match against Australia that began on 15 March 1877 at the MCG in Melbourne.

https://en.wikipedia.org/wiki/Lists_of_oldest_cricketers