Alephee was basically setting a timeline for Eren to achieve certain things. This way, he won't go astray in his quest for unprecedented power.
She elaborated on the topic further.
"You had asked me about soul spells and I had told you it was too early for you to learn them. But I guess now's the time for you to at least know these details.
Soul spells can be seen as the evolution of Hex spells. Cultivating an intent into soul space and then using it to power your spells will make them turn into soul spells. They will not use your mana, only your intent. The stronger the intent, the more powerful your spells will turn out to be.
Since you have an awakened soul sense, you don't need to worry about casting normal Hex spells. The advantages of having a soul sense will gradually become apparent the further you climb up the ranking ladder. So my suggestion would be to start small and start early."
