How old is the book?

The usual academic approach is to assume that the Book of Enoch is not much older than the oldest known surviving fragments. This is the method that produces the "estimate" quoted below in this Wiki article:

The older sections (mainly in the Book of the Watchers) of the text are estimated to date from about 300–200 BCE, and the latest part (Book of Parables) probably to 100 BCE

These dates are simply the age of the Dead Sea Scroll fragments, which don't include any of the Book of Parables - so the parables are then dated to the oldest known Greek example "probably".

The following Wiki link is worth a read and shows that the Book of Enoch was well known and well regarded back in the biblical era:

My own opinion is that there was never a time, since the time of Moses, where you could introduce a book that includes the story of Moses with Moses described only as "that sheep". (See 89.16 onwards) Similarly, Yahweh (Jehovah) is "the Lord of the sheep" and this would only be acceptable if the book was known to be ancient and thought to be genuine.

According to the Book of Jubilees the Exodus began around 1310 BCE. The Biblical account of the fallen angels, and Enoch himself, in Genesis is clearly based on the Book of Enoch. Otherwise, you have to assume that they just threw in an inexplicable angel story, at the beginning of the Bible, for no obvious reasons. So I am sure the Book of Enoch is older than the Torah and was already known in that era.

I believe we can now approximately date "The Flood" because it was the meltdown of the last "ice age" when the sea levels rose globally by about 100 metres. Roughly, it was about 15,000 years ago, and Enoch wrote the book before it happened.

Interestingly, there are some flood details in the Book of Enoch that science is not yet aware of; but that might come to be recognised in the future. One is a "pole shift" and another is how quickly it happened.

There is a book that details the pole shift theory, first published in 1958, written by Charles Hapgood, titled The Path of The Pole, which is still in print. Despite an enthusiastic foreword by Albert Einstein it has been largely ignored by the scientific establishment. My second edition of Hapgood's book, from 1970, contains a foreword by a geologist who describes the reaction of some of his colleagues, to the pole shift theory, as "irrational and hysterical".

The internal structure of our planet is better known now, and I think the boundary between the core and the mantle, which is at 6,000 degrees and is where molten rock sits above molten metal - that could slip.

When a massive ice sheet 3 miles thick, weighing trillions of tons, had built up over North America, the centrifugal force from the rotation of that unbalanced weight eventually caused a slip inside the Earth.

Hapgood presents evidence that the pole was located in Hudson Bay Canada during the last glaciation and then moved to its present location between about 16,000 and 12,000 years ago. Basically, the evidence is that while North America was really cold, Siberia was apparently warmer than it is now.

More recently it has been realised that what is known as "Meltwater Pulse 1B" occurred just at the same time as when Plato reported Atlantis had sunk under the sea. The meltwater pulse was 11,600 years ago at the end of the "Younger Dryas" and Plato reported that his relative Solon had been told by the Egyptian Priesthood in 600 BC that Atlantis sank 9,000 years previously.

There is also evidence being found of an impact from space around 12,800 years ago. Probably several impacts, this is at the begining of the Younger Dryas cold snap, and there are indicators such as melt glass, iridium, and micro-spherules, around the world, in sediments of that age. Some evidence the ice-cap was hit and some theories suggest this may have accelerated the meltdown, and I wonder if the impact might have initiated a pole shift.

Hapgood suggests it may have been a slow movement, whereas I think there is evidence in Enoch (see below) that it only took about one day.
As a result, the ice-cap moved south by about 1,000 miles and then rapidly melted. There are some amazing geological remains in America from the resulting massive water out-flows called the Channeled Scablands.

The part of the Book of Enoch that was apparently written by Noah begins with :-

65.1 And, in those days, Noah saw the Earth had tilted and that its destruction was near.
65.2 And he set off from there, and went to the ends of the Earth, and cried out to his great-grandfather Enoch; and Noah said three times in a bitter voice: "Hear me, hear me, hear me!"
65.3 And he said to him: "Tell me, what is it that is being done on the Earth, that the Earth is so afflicted, and shaken, lest I be destroyed with it!"

There is also this other short and otherwise unexplained section -

57.1 And it came to pass, after this, that I saw another host of chariots, with men riding on them, and they came upon the wind, from the east, and from the west, to the south.
57.2 And the sound of the noise of their chariots was heard. And when this commotion occurred, the Holy Ones observed it from Heaven, and the Pillars of the Earth were shaken from their foundations. And the sound was heard, from the ends of the Earth, to the ends of Heaven, throughout one day.

My explanation is that the North American ice-cap, and America, moved 1,000 miles south in one day, and there was a loud rumbling sound associated with that, and it could be heard everywhere. Probably within a year or two after that most of the ice and the melt-water was in the sea.

There are a number of very ancient megalithic sites around the world, in various places, that are not aligned in a typical north-south layout. It would be interesting to do some research into whether any of these are aligned with Hudson Bay.

Part of the Scablands - the green bits are now wheat fields and all the rest of that landscape was suddenly washed away when the ice melted.