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.