The Chamber of Deputies was five votes short of passing the proposal to the Senate: 303 lawmakers voted for the reduction of the 308 required, while 184 legislators were in opposition.
The measure was opposed by President Dilma Rousseff's leftist's government, the Catholic Church, human rights groups and the United Nations.
But a series of recent crimes committed by minors, including a stabbing death carried out by a teenager in Rio de Janeiro, had galvanized support for changing the law.
The original constitutional change would have reduced the age of criminal responsibility for all crimes, but a modified later version only reduced the age limit for serious crimes such as death and rape.
Lawmakers will still vote on the harsher version of the amendment.