The youngster, accompanied by his mum and gran, cried as he was led into Preston Crown Court, where the legal representatives removed their wigs due to his age.
He sat in front of the dock as the court was told how elderly Stanley Joy was in the flat above his shop, Joys Toys, in Albert Road, Morecambe, when the fire broke out on January 6.
The trader had to be alerted by neighbours.
The ferocious blaze gutted his business - which the pensioner was unable to afford insurance for, due to the recent floods - and destroyed a jewellery shop next door.
The court heard fire investigators found two separate seats of fire in the shop.
The boy had stolen BB guns, knives,and assorted toys and the guns were recovered from behind a Morrisons store, where he had stolen alcohol.
The judge refused an attempt by the press to reveal the Morecambe boy's identity and imposed an order protecting the 15-year-old, who was found guilty of two counts of arson, burglary, theft and assaulting a PC after a trial.
The youth was made to sit in the dock for his sentencing.
Justice William Davis said: " This, what you did, put people's lives at risk. The elderly man could have been trapped.
" If you were an adult and had done what you did you would have faced nine or 10 years in prison.
"You are not a fool by any means, you are an intelligent young man and you understand things. I hope you understand this sentence is intended to reflect the fact you have done something very serious."
His 16-year-old accomplice, who also cannot be named, was given a curfew and youth rehabilitation order in connection with the same case at Lancaster Youth Court last week.