I have an article by Leslie McFall from the Journal of the Evangelical Theological Society (53/3, sept. 2010, pp. 375-533), carefully and deeply analyzing the chronologies of Saul and David. The author places the birth of Absalom in about 1006 BC, putting the birth of Amnon in about 1010-1009. The article places the rape of Tamar in 989 BC, two years after David's affair with Bathsheba. If we go with 1009 for a birthdate for Amnon, we can play with round numbers and say he was 20 years old in the rape story of 2 Sam. 13. The age of Tamar, however, is a complete unknown. We have absolutely zero information about her birth or birth order. We know she was Absalom's sister, but don't know whether she was even older or younger.

I think your analysis of her age doesn't take us anywhere. Daughters of the king were usually given in marriage to others as political alliances, not in normal community wedding scenarios, so her presence in the house doesn't necessarily tell us anything about her age. She was still in the palace. She was old enough to carry some weight in adult-matter conversation would indicate she was older than 12 (remember their patriarchal culture). She was old enough to make bread from scratch, but all young women were trained in the culinary arts from their youngest years. I don't think any of these clues give us an indication of her age. At 20 years old, it would seem to me odd that he went ga-ga over a 12-year-old, but hey, we all know that such things happen.

As far as "such a thing should not be done in Israel," Tamar is wise beyond her years if she's only 12. What does she mean by her phrase? Rape is a serious violation of the morals and customs of Israel and threatens the very fabric of society. In Genesis 34.7, a similar scenario, it is described as sacrilege. Here Tamar identifies is as being clearly wicked, which would indicate a sin against God and the community. The community of Israel lived by the Lord's covenant, and such an ill-mannered, self-seeking, desire-thirsty, politically destructive, and community-weakening breach of the covenant was to be avoided. Her warning mirrors God's in Gen. 4.7: "If you don't do what is right, win is crouching at your door; it desires to have you, but you must master it."

Her age is not material to the story. What is material to the story is:

1. The perversion of David's household as a mirror of his own sin exemplified in the affair with Bathsheba.
2. The effects of "private" sin bring the downfall of person, family, and society. As Dewey Bertolini says, "Secret sin on earth is still open scandal in heaven."

2 Samuel 13:12 sees Tamar appealing to Amnon, "such a thing is not done in Israel." It is easy to read the story and be appalled by the thought of Incest in David's family. But it doesn't require too much thought to recall that there are several instances of incest in Israel. Abraham and Sarah had precisely the same blood relationship that Amnon and Tamar had. Israel's first judge was descended from Caleb through incest. Tamar herself even suggested that genetics was not the issue when she (paraphrase) told Amnon, "If you really want me, then do it right and go through my father. He will cooperate with you." This suggests that if Amnon would give her a chance to ripen, then she would go along with him.

But for all Israel sinned against God, pedophilia was never mentioned unarguably. (I'm aware of Numbers 31.) Pedophilia was one sin Israel kept clear of without God's intervening through Moses. We never hear of God being angry at Israel for raping underaged women. And even though the women married younger, that does not necessarily mean they consummated during pre-pubescence.

I only present the case that pedophilia was the thing "not done in Israel" because the case for incest is obvious. But I'm not declaring it so.

We can't date the incident precisely. But there are clues. David reigned from Hebron from 1010 BC to 1003 BC, and at age 37, he moved to Jerusalem until he died at age 70. While in Hebron, David had six sons (2 Samuel 3:2-5). For sake of discussion, let’s assign one son a year from 1009 BC to 1004 BC. (The only thing that matters is Amnon's year of birth which was likely not far after 1010 BC.) That would place Amnon as born in 1009 BC and Absalom as born in 1007 BC. As of 1003 BC, Tamar was not yet born. (The whole post breaks down if Tamar was born, but was omitted from 2 Samuel 3:2-5 because she was a girl.)

Forward to 989 BC. The account of 2 Samuel 13 tells us that Amnon was old enough to have moved out of the palace, but young enough that his father would visit during a minor illness. And Tamar was young enough still to be living under her father’s close direction - David could still dictate how she spent her time without regard to other plans she may have had. But she was old enough to carry some weight in an adult-matter conversation; and also old enough to be able to make bread from scratch unsupervised. Finally, Tamar was old enough that when Tamar was conceived, David still had an interest in Maacah, the mother who gave birth to Absalom in 1007 BC (it could have been later). I suggest Tamar was born in 1001 BC, making her 12 in 989 BC.

Whatever was "not done in Israel" was correctable if Amnon would just do things the right way. Age was correctable; genetics was not.

I am not persuaded by the observation that 12 was marrying age. Perhaps it was. But what does that prove? Our culture is so accustomed to couples leaving their own reception in a hurry to get alone that we merely assume that all couples once married place a high priority on consummation. I suggest that Joel 1:8 proves that is not the case. The couple would know when they are ready to start. But Joel presents the most miserable circumstances he could construct. A woman of youth, whose husband had died before consummation, leaving her as "spoiled goods."

Fire away.