Particularly giant or multiple lesions usually present at birth but can be shortly after.
Number of naevi increases through childhood, but high volatility, eg over 3yr period, about a third change pattern, sun burn increases number. Halo pattern of disappearance (with depigmentation) rare.
Can be associated with neurocutaneous melanosis; if suspected then neuroimaging is advisable, seeking evidence of leptomeningeal melanotic lesions.
Malignant change in childhood is possible, but only really in lesions greater than 1% BSA size. Melanoma detection relies on morphology eg ABCDE criteria (asymmetry, border irregularity, color variegation, diameter (6 mm, and evolving). ABCDE very low specificity in kids but melanoma does present in children (and pigmented naevi are risk factor). Dermoscopic changes in blood vessels important.
Beckers naevus tends to present at puberty, usually torso or upper arm, irregular, hairy.