Aftab Bahadur was hanged in Lahore's jail early on Wednesday morning. Bahadur, a Christian, had been convicted of a double murder in the city in 1992. Campaigners called his execution "shameful".
The death sentence could be passed on 15-year-olds at the time, although the minimum age was raised to 18 in 2000.
Reprieve said Bahadur had been convicted on the basis of testimony from two eyewitnesses. They both later retracted their statements, saying they were made under torture
“Pakistan proceeded with Mr Bahadur’s execution despite his having been sentenced to death when he was a child - in violation of both international and Pakistani law,” Reprieve said..
His letter thus: “But I do not know whether that will make any difference,” he wrote. “I have not given up hope, though the night is very dark ... It would perhaps have been better not to have to think of what the police did to try to get me to confess falsely to this crime.”