He alerted the service to this flaw and was paid $10,000 as a reward. Instagram said the vulnerability had been fixed.
"I would have been able to eliminate anyone, even Justin Bieber comments from there," Jani said, according to a Google translation of the Iltalehti article.
Facebook operates a "bug bounty" program that rewards those who report vulnerabilities in its services, including Instagram.
Facebook has rewarded more than $4.3 million to more than 800 people under the program since it launched in 2011.
Jani plans to spend his money on a new bicycle and a football. According to Facebook's own rules, he is too young to have an Instagram account, for which the recommended minimum age is 13.