I think the last time realm first achievements were a thing (for anything other than raids) was in Lich king, but I might be mistaken.

Since I wrote my previous comment, I thought about it, and there are advantages for starting earlier. Reaching max level gives you access to world quests, which can be a source of gear and reputation. It also leaves you more time to complete the weekly objectives before the next reset. If I recall correctly, last time (the launch of Dragonflight) you got an item you can use to increase the power of a piece of crafted gear.

For the average player it doesn't really matter, but for the world first racers, it might.