Training must begin before the 19th year. In particular, the shuna need to be able consciously hold the spark. Once they learn to do that, there is no danger of the spark dying out. Typically, training begins at the age of 16 and above in the White Island. The princess is probably already preparing for the journey. 

Without being able to consciously hold the spark, the shuna has no control. Only in very rare moments of intense stress will it emanate in random bursts. Otherwise, there is nothing. There are instances of the shuna feeling an affinity towards certain elements even before they are trained, but it doesn't amount to anything but an awareness of, say, a nearby water body or the ability to sense distant storms