its just a dimmer of how 'on' it is
The first trip into the madness place is the definitive point at which one may be called a Spark. Prior to breaking through, the individual may have not shown any particular signs; afterwards, there's no turning back. This is usually traumatic for the Spark ✣ and dangerous for bystanders, since breakthrough devices are notoriously destructive (with few exceptions). Most Sparks break through in their teens or later. ✣
Young Sparks usually won't survive without powerful protection ✣ . If the breakthrough device does not kill its creator and everyone in the general vicinity, and the new Spark also survives the retribution of other survivors, he or she becomes part of the incessant power politics of the Gifted, usually as the pawn of an existing power who noticed the chaos, but sometimes (especially with appropriate protection and mentoring) they become players in their own right.
Agatha is a unique case as she did not really break through, the pendant she wore at the time easing and suppressing many of the Spark urges in such a way that she was more 'eased through' rather than break through.