The killer, aged 15 at the time, knifed Salum Kombo in the chest. Judge Nicholas Loraine-Smith, sentencing him at Southwark Crown Court in central London, said: "There was nothing brave about what you did. This was quite simply an act of cowardice, as so many stabbings are."
Mr Kombo, 18, bled to death near the scene of the attack in east London last December.
David Jeremy, QC, prosecuting, said a previous friendship between Mr Kombo and the defendant had turned to "hostility".
He said: "In the days leading up to 20 December, there had been a trade of insults and threats between these two boys, both directly and on Facebook. At one stage the boy who died called (the defendant] a pussy. That insult appears to have particularly incensed him."
The court heard that on the night of the murder the killer had been with friends who tried to calm him down when he became angry and started talking about stabbing the older youth.
When Mr Kombo turned up, the two teenagers walked off, apparently to settle their differences, when the younger boy pulled out a knife without warning and stabbed him to the upper chest.
The boy, who cannot be named because of his age, later admitted the stabbing, but he claimed he was acting in self-defence. An Old Bailey jury rejected his explanation and convicted him of murder.