They mentioned in the Reddit AuA that the scaling will be like scaling in Cyrodil. A low level player can go to a new adventure zone and will have his health, magicka, and stamina scaled up to match the level of mobs in the zone. However, just like in Cyrodil, a scaled up low-level will be less effective than a high-level player because of skills and now champion points. It means that a level 25 player can now run a trial without getting one-shot by trash but that player will still be a liability to the group on a DPS/HPS basis.