Kyle Sheehan was stabbed in the leg in Jiggins Lane, Bartley Green, at about 22:15 BST on 21 September.
A man, 23, has been arrested on suspicion of murder. Three people, aged 16, 18 and 29, arrested on suspicion of wounding have been released on bail.
Kyle had been in intensive care but died on Friday night, West Midlands Police said.
Det Ch Insp Wayne Jones: "He leaves behind a family and many friends who are distraught and struggling to come to terms with what has happened.
"We know many people were in the area and would have seen what went on that night but many are still yet to come forward.
"We appeal to you to speak to us and help us find those people responsible for killing Kyle."

The Broncos needed to better Salford's result at Batley to keep their hopes of a Super League return alive, but the Red Devils won 42-14 to finish fifth.
Rhys Williams scored three of London's nine tries against Featherstone, who lost all seven games in the Qualifiers.
Both clubs will play in the Championship again in 2017.

They have put up wanted posters in hostels around the town, which is hosting the Winter Olympics next month.
The woman, 23-year-old Ruzanna Ibragimova from Dagestan in the North Caucasus region, is believed to be the widow of an Islamist militant.
She is thought to have slipped into Sochi earlier this month, bypassing tight security ahead of the Games.
At least two other potential female suicide bombers are also at large in the region, other police posters state.
Russian President Vladimir Putin has launched one of the biggest security operations in Olympic history, and has personally inspected the sites.
More than 30,000 police and interior ministry troops have been deployed and access to the area limited.
Security fears were heightened after two suicide bomb attacks killed 34 people in the southern city of Volgograd on 29 and 30 December.
US President Barack Obama, in a telephone conversation with the Kremlin on Tuesday, offered America's "full assistance" in making the Olympics "safe and secure", the White House said.
Two US warships will be on standby in the Black Sea when the Games begin on 7 February. Washington has also offered to supply Russia with hi-tech equipment to help detect improvised explosives.
Islamist militants from Dagestan and nearby republics of Ingushetia and Chechnya are considered by Moscow to be a major threat to the Games.
Several national Olympic associations have reported being sent an email threatening athletes with attack, but it was widely dismissed by the International Olympic Committee (IOC) and national bodies as not credible.
The IOC said it appeared to be  "a random message from a member of the public," and posed no threat, but repeated that it "takes security very seriously".
Officials at the British Olympic Association said they "receive correspondence of every type and it is not uncommon to come across something like this that lacks credibility".
"It is extremely important in matters such as this that everyone maintains a level head and a sensible perspective," said spokesman Darryl Seibel.
A 16-year-old boy who was stabbed in Birmingham has died, prompting a murder investigation.
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London Broncos thrashed Featherstone in their final Qualifiers match but Salford's victory meant they missed out on a place in the Million Pound Game.
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Russian police in Sochi are hunting for a woman they fear may be planning to carry out a suicide bomb attack.