How old would you say are the European ethnicities?

[Creoda]

That is, what length of time have modern ethnic groups been essentially the same people and distinct, primarily genetically + identity/culture wise?

IMO:

English: at least 1300 years, since the time of Bede
Scottish: 1000-1100 years
Welsh: Antiquity as British, 1500 years as specifically Welsh/Cymric
Irish: Antiquity, up to 4000 years potentially
Germans: Antiquity, perhaps 2300 years, after they spread South into Celtic lands.
Dutch: under 500 years? Maybe a few centuries older as Netherlandish.
White Americans: 250-300 years (Old Stock), perhaps less than a century for modern White Americans overall.
White Australians: 100-200 years for Anglo-Celtic white Australians.
Greeks: Bronze Age, 4-5000 years?

French is tough to say, but I wouldn't put it back as far as Gallic times, nor as recently as the French revolution.

[de Burgh II]

As you reiterated very succinctly, Europeans were more tribal in nature before nationalistic, modern day terms came into fruition. After the dissolution of Empires, Monarchy, eventual collapse of conjoined nation-states; it wasn't until recently European nationalities gained prominence after the rising interest in capitalistic, self-autonomous countries in our globalized economy today that gained momentum during the Industrial Revolution era and onwards.

General Speaking, I would say between 1000 to 1930 C.E. when European countries formed their nationalities during the formation of their Kingdoms and for others, during the collapse of Empires such as Austria-Hungary, Holy Roman Empire, etc.