Is Marfans discovered @ an early age w/ symptoms seen early or can abnormal tallness come about later in life like almost 2 decades after puberty ends?

May be later. You are born with the condition but manifestations may vary. Diagnosis may not be made till adulthood or not until a serious complication occurs. See this review:http://www.uptodate.com/contents/marfan-syndrome-the-basics?source=search_result&search=marfan+syndrome&selectedTitle=1%7E105.