Some doctors are just not that good, and cannot produce OE pregnancy for their patient, so they start to blame your age, your ovary, your FSH, but never themselves. When other doctors can do it, they diminish it to "DE".
The truly inspiring doctors, like the one I currently have, will have the intellectual curiosity to push the limit. My RE wants to see a 50 achieving OE. He constantly challenges himself on the oldest woman he can get pregnant and tweaks his protocols all the time.