David Lamont had gone outside to confront a group of people who were making noise shortly after midnight on Sept. 21, police said.
Lamont ended up in a confrontation with the people on Middleton Place outside his home. When police arrived, he was found lying on the street with a head injury, police said.
Lamont was taken in critical condition to Eden Medical Center in Castro Valley, where he spent the next several weeks receiving treatment for his injuries.
A few days after the incident, police identified an 18-year-old person of interest in the case who they said had been present during the assault. Police said there were three other juveniles present as well.
But after investigating, police said Friday that the Alameda County District Attorney's Office has declined to file any charges, citing a lack of evidence.
Police said in a statement that Lamont's family wants to thank the community for their support and "would simply like to put the incident behind them and continue to focus on healing."