Dai Renai: Boku wo Wasureru Kimi to (2018)

Romance and struggles between a couple hit with incurable disease. How they try to create and maintain the precious memories even after one has left for afterlife.

Both main FM and M have age gap
They have children, in BS its only amnesia
Well the story also develop quick for the romance.

Dai renai touches on the Alzheimer's Disease topic as well though it explores more on the young onset reality