Jitu Patel, who ran the Supamac store in the city's borough of Melton, had bravely stood up to the attackers using a parcel containing a long metal pole to defend himself on May 30 last year.
Kevin Francis-Smith, 18, denied the attempted robbery charge but was convicted by a jury at a recent trial and sentenced to two-and-a-half years in jail at Leicester Crown Court on Friday.
His 14-year-old accomplice - who cannot be named for legal reasons - had joined him in demanding cash from the till at Patel's store and pleaded guilty to the offence and was given a two-year youth rehabilitation order with supervision, and a 30-day specified activity requirement.
Sentencing the teenagers, Recorder Graham Huston said the knives were "large, dangerous and menacing" and waved dangerously close to the victim's face.
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"That man had served the local community, providing a valuable service for many years, and worked extremely hard throughout his life. In the months leading up to his retirement he didn't deserve to be treated in such a violent and mean way.
You two thought it would be an easy push over, but Mr Patel proved to be no push over and he was prepared to chase you two cowardly individuals from his shop. He was left shaking like a leaf and genuinely believed he was going to die," Huston added.
Patel and his wife, Ila, retired last September and the shop is now a One Stop store.