If seborrheic dermatitis is related to genetics, why didn't i have any signs or probs w skin as a child? it showed up suddenly and aggressively.

Onset of derm: It is likely that your underlying predisposition to these skin issues has been activated by a trigger or combination of them. Skin ailments in the atopic realm can be aggravated by a large variety of instigators including environment, hygiene products, allergies, illness, hormonal changes, stress, diet, medications, etc. etc. etc.