How old is the book?

The usual academic approach is to assume that the book is about a week 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, 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.
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 50 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 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 contains a foreword by a geologist who describes the reaction of some of his colleagues as irrational and hysterical.

Hapgood presents evidence that the pole was located in Hudson Bay Canada during the last glaciation and moved to its present location between 18,000 and 12,000 years ago. Hapgood suggests it may have been a slow movement whereas I think there is evidence in Enoch (see below) that it only took one day.
The interior of our planet is extremely hot and is a liquid. The core is a ball of mainly molten metal the size of the planet Mars and above that is molten rock almost up to the surface. When a massive ice sheet, weighing trillions of tons, built up over north America, the centrifugal force from rotating that unbalanced weight eventually caused a slip to occur inside the planet. As a result the ice cap moved south by about 1,000 miles and then rapidly melted.

The part of the Book of Enoch that was added 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 a short and otherwise unexplained section at the end of Enoch's second parable :-

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 it that could be heard everywhere.

There are a number of 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 point towards Hudson Bay.

There was a short return to cold conditions about two thousand years after the flood, in an event now called the Younger Dryas, which seems to have been caused by comet impacts around 12,800 years ago. There is a recent paper about that here -
The hazard from fragmenting comets - W.M. Napier