St. Gregory Palamas said, as I recall, that the Theotokos achieved theosis at the time of her Entrance into the Temple at age 3. Her ascending the steps to the temple and being brought into the Holy of Holies was both a physical depiction of her inner ascent and a recognition of her state of theois.
He said that at the time of the Annunciation until her giving birth, the Theotokos was in the highest state of union of God possible, since her flesh and blood and Christ's flesh were as closely entwined as is humanly possible.
He stated that it is this period of 9 months that places the experience of theosis that the Theotokos had at a level that no other human will ever experience, since no-one else will ever be physically joined to God like that.
This is why, although she is "just" a human like the rest of us with no special prevenient grace or anything, that the Theotokos has such an exalted place in our veneration of the saints. There was never any question, from the earliest days of the Church, whether anyone would come along later who would be a more God-filled saint than she.
"St. Gregory Palamas said, as I recall, that the Theotokos achieved theosis at the time of her Entrance into the Temple at age 3. Her ascending the steps to the temple and being brought into the Holy of Holies was both a physical depiction of her inner ascent and a recognition of her state of theois."
Are you thinking of this, from his sermon on the Feast of the Entrance of the Most Holy Theotokos into the Temple?
"Now, when Righteous Joachim and Anna saw that they had been granted their wish, and that the divine promise to them was realized in fact, then they on their part, as true lovers of God, hastened to fulfill their vow given to God as soon as the child had been weaned from milk. They have now led this truly sanctified child of God, now the Mother of God, this Virgin into the Temple of God. And She, being filled with Divine gifts even at such a tender age, ... She, rather than others, determined what was being done over Her. In Her manner She showed that She was not so much presented into the Temple, but that She Herself entered into the service of God of her own accord, as if she had wings, striving towards this sacred and divine love. She considered it desirable and fitting that she should enter into the Temple and dwell in the Holy of Holies."
Notice how +Gregory stresses that she did these wonderous things "...of her own accord...."