The state court in Stade, Lower Saxony, rejected calls from both the state prosecutors and the defence lawyer for Ernst B. to be found not guilty of manslaughter.
But he was given a nine-month suspended sentence, meaning he will not go to prison.
The teenager broke into the man's home in Sittensen with four other masked men in 2010, threatening him and demanding money.
But when his burglar alarm went off, the pensioner was able to get free and grab one of the guns he kept in the house.
As the raiders fled, he fired several shots, hitting the 16-year-old fatally in the back.
The rest of the gang were jailed for between three-and-a-half and four years.
The dead boy's family spent years pressing courts and prosecutors to take on the case, which sparked a debate in Germany about the limits of reasonable self-defence.
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