A teenage boy with autism is missing after fleeing the scene of a fire in Sydney's west.

A small fire began in a home on First Street, Kingswood, about 5.30am (AEDT) on Friday, police said.

It was extinguished and an elderly man and woman were treated for smoke inhalation.

However, 17-year-old Nathan Gent, who is autistic, fled the scene and police have concerns for his welfare.

He's described as being 183cm tall, with a thin build and brown hair.

Anyone who sees Nathan is being asked to contact police.