The consensus is that Ethelred died between late November 1093 and some time in the year 1098, and that, whether because of his youth or for some other reason (like a religious vocation), he had no children.
His birth date was *probably* between 1072 at earliest and 1075 at latest - 1072 would require that either Edward and Edmund were twins, or Edmund and Ethelred were.
He was given Auchmoor and (probably) other territories at a young age (Lawrie XIV and Notes, pp. 11-12, 243-246). It is likely that he was made Abbot of Dunkeld and Earl of Fife at about the same time.
Because Dunkeld was a *lay* abbacy, it was not subject to normal ecclesiastical rules: the abbot did not have to take holy orders or be celibate (Crinan, lay abbot of Dunkeld, d. 1045, certainly was not), and perhaps the normal age requirement was waived as well. (This did not mean that someone who held the position could not develop a religious vocation - and Ethelred, as a son of Saint Margaret, was particularly likely to have done so.)
Ethelred is conspicuous by his absence from the tumultuous events of Malcolm III's reign, *and* from the succession wars that followed. When Malcolm marched south for the last time, he took his eldest son Edward and a probably-younger son, Edgar (who could not have been more than fifteen or sixteen, at most, and may have been as young as fourteen). He *probably* took his second son, Edmund, though this is nowhere stated. But apparently he did not take Ethelred.
It wasn't because a lay abbot wasn't allowed to fight - Crinan did. Was there something about Ethelred - a vocation, a physical flaw? - that fitted him better for the cloister than for combat?
(Fans of the Deryni series by Katherine Kurtz are likely to think first and foremost of club foot, a relatively common birth defect. This would seriously affect Ethelred's martial activity, making long marches (for instance) difficult or impossible, but would not prevent him carrying out his duties as abbot.)
Circumstantial evidence suggests that while his father and brothers were marching toward their rendezvous with destiny, Ethelred was attending his ailing mother, who had become increasingly frail due to excessive fasting and prayer. It is said to have been Edgar who rode pell-mell to Edinburgh to tell Queen Margaret that she was now a widow with one less son - at which point she gave up on life and joined them within hours or a few days. But it was apparently Ethelred who took charge of the funeral arrangements (probably *not* complicated by the usurping Donald III "Bane"). It would not be surprising if he caught his death of pneumonia from leading the cortege in the chill of a Scottish winter.
As to Edmund, with his father and older brother dead and his younger brother sent off as a messenger, he may have had his hands full trying to keep a strategic retreat from turning into a full-scale rout. (*Somebody* had to be in charge, and he was next in line.)
While we don't know for sure when the "short form" of Ethelred's Lochleven charter was written, we can hazard some guesses. He had to have been at least fourteen, so no earlier than 1086 (and more likely 1089-90). He does not specifically refer to the souls of his parents, but those of his ancestors in general. His brother Edmund is there as a witness, which he would not have been after November 1094, and perhaps not even in late 1093. Perhaps it was drawn up during preparations for that last march south, with Ethelred having "a bad feeling about this".