Oldest as in who lived the longest? Or rather who was the first pharaoh?
Who lived the longest? You will never get an accurate answer really. There's stories spread everywhere that Ramesses was the oldest pharaoh, being nearly 100 years old. Yet, it is supposedly recorded that Pepy II ruled for 94 years and lived to be 100. Which, by the way at that age he would have been pharoah at age 6, taking Tutankhamun's title as "Egypt's youngest pharaoh."
As for who was the first pharaoh, that would be King Narmer (aka King Menes). He united the Upper and Lower Egypt in 3100 BC.