Sephiroth is five year old when his daemon starts mimicking the daemons of people they are around. Which is a normal stage of development in children, perfectly on schedule.
What's not normal is that when this happen his daemon doesn't flit back to something else in a matter of minutes, or, if interacting directly with that person and their daemon, as soon as they have left the room.
Sephiroth has always had a surprisingly long attention span for a child. It shouldn't be called abnormal, it should be called remarkable. The lab hands are impressed, the caretakers find it adorable. Professor Hojo has not a word of criticism about it; if any, when he sees a little tonberry toddling after the boy he even tends to smile.
Sephiroth's daemon can stay a tonberry for days after a visit from Hojo, and Gast would be all ready to hail it as proof of developing parental bonding if Sephiroth's face didn't go so utterly blank.
When he moves, he mimics the tonberry, too.
Gast gets the scalpel back before he can swing it down into his own daemon's flank. But try as he may he can't convince Sephiroth to get his daemon to stop mimicking people's.
He starts bringing the boy in his office when he has paperwork to do. It doesn't stop the behaviour but at least when his daemon rolls around as an otter with Gast's Shelag, Sephiroth actually smiles.