Step 1: Achievement of urinary continence is a developmental milestone, and a child is not labelled enuretic until they pass the age by which control is normally gained.
Step 2: For daytime control the accepted threshold is around 4 years, but the bladder takes longer to stay dry overnight.
Step 3: Night-time wetting remains within normal limits until about 6 years of age, after which continuing involuntary voiding during sleep is considered pathological nocturnal enuresis.
Step 4: Since the question asks the age up to which nocturnal enuresis is still normal, the correct cut-off is 6 years.
\[\boxed{\text{6 years}}\]