Urine leakage or having «accidents» also signal problems, either with infection, loss of sphincter control, or another underlying disease. (thesprucepets.com)
Starting earlier than 18 months, the less likely your child will even have proper sphincter control. (pottytraining-help.com)
Reflux happens secondary to inadequate sphincter control (valves that shut off the top of the stomach and top of the esophagus) due to immature nervous systems. (abcdoula.com)