an old man is twice a child
Proverb meaning that elderly people can become childlike in behavior or need care similar to that of a young child.
My grandfather has become selfish lately, but as the saying goes, an old man is twice a child, so it can't be helped.
Literally “at sixty, a three-year-old child,” comparing an elderly person’s behavior or dependence to that of a small child. 三つ子 here means a three-year-old, not a triplet.