I think SORN applies once the vehicle has been taxed under the new regime, and applies regardless of the age of the vehicle.
The tax status for a vehicle which was untaxed before SORN came in is normally "Unlicensed" and this continues until you get a Historic Vehicle tax disc from a DVLA office. From then on, if you need to lay the bike up and the tax / MoT / insurance run out, you need to declare SORN.