Dual Spec may unlock before level 40
Dual Specialization in WoW Forever unlocks at level 40 for now, but the developers are discussing an earlier level. Josh Greenfield said so in the third episode of The WoW: Forever Podcast. Level 30 came up, half as a joke.
The remark came after about 45 minutes, in a talk about tanks in dungeons. Original dungeons could be tanked by almost anyone, Greenfield said: a Warrior with a two-hander, or a Druid in 
Bear Form without much in Feral.
One of the benefits we have is having dual spec, which will come online at level 40 currently. We’re talking about what the right level for that to actually enable is, but currently it’s set to 40.
Maybe a cheeky 30. I don’t know. We’ll see.
Dual Spec also gives the team room for harder content, the developers added. With two specs, a dungeon can expect a dedicated tank or healer from that point. The early dungeons, such as Ruins of Lordaeron and Wailing Caverns, never needed a specific spec.
At level 40, Dual Specialization costs 50 gold, which many new players find a lot. Whether the level changes before launch, and to which level, Blizzard has not decided.
Source: Wowhead