Unlike the other orders Shallan only had to speak one oath and even then i believe that at the time of her mothers death Pattern may have materialize into her Shardblade out of need, if we compare it to Kaladin's journey to Radiance then isn't his powers also manifesting during great times of need so i think this was the first time summoning her Shardblade but her powers of observation and her ability to create works of art were what drew the unwanted attention of her mother in the first place
On the matter of age It is more i would think these Proto-radiants would have been like a squire in medieval times, and if that is the case it holds precedence, unless the situation was almost completely lost they would not have needed to fight. Instead they would be trained in the keeping of their oaths and then using these marvelous powers in accordance to those oaths. Also in medieval times squires were chosen closer to the ages of 8 -9 not 11-12 do to the fact the it would take years to reach knighthood and most would be reaching that point around 15.
saying I am not a history major but an engineering major who dabbles in art and writing who has come into this discussion lateand is catching up on the replies.