Can adults get umbilical hernias?

Yes. Umbilical hernias are quite common in adults. Very often, a small hernia may be present at birth which may temporarily close off. In adulthood, stress to the abdominal wall that may occur with pregnancy, weight gain, excessive lifting, etc., may lead to this hernia re-forming.
Yes. Umbilical hernias can occur in both children and adults.
Yes. Women who become pregnant. Men who develop beer bellies , anyone who does a lot of straining lifting. Smokers who cough a lot. Usually 40's to 50's they begin to show. Most often seen in adults though some babies may born with them. If painful or growing have it fixed. If stable for years and not painful , don't need to fix it. .