The Ship of Theseus asks when something stops being itself

The Ship of Theseus paradox, discussed by Plutarch in 75 AD, asks: if a ship's planks are gradually replaced one by one until no original plank remains, is it still the same ship? Thomas Hobbes added a wrinkle: what if someone collected all the original planks and assembled them back into a ship — which one is the 'real' Ship of Theseus? The paradox applies to identity generally. Every atom in your body is replaced over roughly seven years; most of your childhood memories are reconstructions. Whether you are 'the same person' you were at age 10 is not trivially answered.

Source: Plutarch, Life of Theseus (c. 75 AD)