Is there an age at which it's normal to be unable to get an erection?

See below. The prevalence of ED is increases with age. One notable study showed that ~40% of men are affected at age 40 and ~70% of men are affected at age 70. A good estimate is that the ~% of men affected by ED is equal to one’s age. With age come increasing risk factors for ED: diabetes, high blood pressure, heart and vascular disease, cholesterol, medications, neurological and kidney disease, low T, etc .
No. Loss of erection at any age is troubling and needs evaluation to ensure no serious medical conditions are also present.