Think mild. If you are a developing fetus and share your mothers first time exposure/infection, it is deadly. If you don't die/abort, it can leave you blind/deaf/with organ and brain damage at birth. If you are a toddler through adulthood, think mild problem. The lifelong issue with rubella would be from fetal exposure/disease.
If you are a fetus! Adults and children contracting rubella generally are mildly symptomatic with a viral syndrome that may also have a rash, however pregnant women can have profound infections of their fetus (es) that may lead to pregnancy loss, permanent cardiac, neurologic, ear/eye problems and poor fetal growth.