God originally determined 30 years as the ideal span of life for all animals, including mankind. The donkey, the dog, and the monkey considered it much too long, however, and begged God to reduce their years by 18, 12, and 10. Being healthy, vigorous, and somewhat greedy, the man asked to be given those extra years.

God agreed, so man’s years totaled 70. The first 30 are his own and they pass quickly. The next 18 are the “donkey years,” during which he has to carry countless burdens on his back. Then come the “dog years”…12 years when he can do little but grow and drag himself along. This is followed by the “monkey years,” his closing 10, when he grows rather strange and does things that make children laugh at him.

The Duration of Life, from Grimm’s Fairy Ttales.