It is normal for the height difference between grandchild and grandparent to decrease as both grow older.
Grandchildren get taller and with age most grandparents lose at least a little height.
It is not unusual to lose an inch or so in height as an older adult. But if a person has osteoporosis the shortening continues. According to Dr. Barbara Lukert, at KU Medical Center, osteoporosis is condition that results in thinning bones. Bone density decreases with osteoporosis. As the bones get more porous, they become more fragile.