What reasons might a 5yo trachee develop o2 dependence after 5yrs stable on room air? Expreemie

Ask subspecialist. A parent of a preemie should never hesitate to ask the local doctor of the child for a referral if a problem develops. If there is a major medical center or children's hospital nearby, there may be experts experienced in helping a preemie who develops later problems related to birth or treatment in nicu.
Several reasons. I would discuss this with the physician who is managing your child's tracheostomy tube. Sometimes this can be due to an issue at night, like mild obstruction, causing an oxygen need at night. Sometimes a granuloma or small scar tissue develops in the airway around the trach site and causes a small amount of blockage. Sometimes there can be a mild inflammation of the airway from a virus.
No simple answer. Possible reasons why a child with a trach might need extra oxygen after being ok on room air for a while . . . His lungs did not grow enough, compared to the rest of his body; mild inflammation is going on in the lungs, making it harder for oxygen to diffuse into the blood; he is not sighing as much, so more air sacs stay collapsed; he is more anemic, so has less hemoglobin to carry oxygen; etc...