Why might someone have poor bladder control at a young age?

Depends. There are lost of reasons for this mandy. How old is young? Boy or girl? Is this only at night or night and day? Are they having accidents, frequency, urgency, all ofmthe above? Any urinary infections? Is this person pooping every day?
Bladder . It depends what’s person’s age, cause of poor control , Under what circumstances control is lost , sex of person , what other medical problems are there . See your family doctor . .