Is it possible for congenital heart defects (one or more) to be diagnosed later on in life?

Depends: This depends on the defect. Some can hide and some do not.It is common for someone to go a whole lifetime with a minor issue like a patent foramen ovale (some don't consider it a defect).Some issues like a trivial leaking mitral valve are only evident because of advanced technology (echocardiogram).Other issues like a misdirected plumbing or holes between chambers are found in childhood.