Try looking on the back of the hull fairly high up, most builders stamp a number on the moulding. From that you should be able to work out the year. i.e. if it said something like xxxx/88 that would be a 1988.
We bought a Shetland 570 way back in 1983,(our first boat and we loved it so!)) it was about two years old then. By that time the 570 had been renamed as the 'Kestrel' by Shetland if I remember rightly, so if it is a '570' it's going to be at least twenty years old by now. I think they were manufactured at Diss in Norfolk, but can't think where the serial number was after all this time, which would give you a clue to its age.
can't really help with age except that the 570 was produced from about the late sixties or early 70's until the early 80's. They continued under different names ( not 570) with gradually upgraded specs until much later. Shetland have gone bust at least three times so finding records of your boat may be difficult for the present incarnation.
I had a 570 in the early eighties and mine was pretty old then. She had a 70hp Evinrude initially but that engine proved a bit tired and depending on loading could struggle to get her on the plane. I replaced that with a 115hp Yamaha and the boat flew, topping out at circa 40 knots. That was little overpowered as the max hp recommended was 100hp and she tended to porpoise a bit at speed and no amount of trimming would sort it.
She was a super little sea boat and very strongly built. I was out in conditions I would not try nowadays in my flybridge cruiser which is double the length and circa 10 times the displacement!
I have fond memories of that boat and I wish you every luck with yours.