And I forgot

9) (though it actually goes after 5, as it is another pragmatic, not ideological reason) It creates a feedback loop. Daughters whose mothers were themselves young mothers, tend to in turn become young mothers. This effect remains strong even if they were themselves procreated at a later age. And the feedback loop can work in reverse too; as a matter of fact it is part of the reason for demographic suicide. After WW2, increasing numbers of women fully entered the labour force. They increasingly delayed their first pregnancies because of that. Their daughters in turn further delayed them, and so on. You end up with the negative long-term growth of Japan, Spain, Italy, East Germany, mainland China, Cuba, and, yes, Iran too. In this case the feedback loop would mean that the more young mothers, and the younger some of them are, today, sets up the scenario for all their daughters (whether procreated at 14, 24, 34 or 44) to be more likely to in turn become young mothers, and some of them even younger than Mom was. This effect, of course, will only be perceivable when the daughters of these young mothers reach fertility themselves. But it will exist - and reinforce all the other trends.