I just started The Plaza of Conquests at level 40, and most of my friends are in there too, but are like 43 and 44. Or does it really matter?
It will later on if you complete Dragonspyre too early. You must be level 50+ to move on to Celestia so you will be stuck doing side quests to level up. Take your time, and do those side quests now to level up more.
That's not exactly true, grizzleheim gives almost no XP.
No it doesn't, but you get through it and hit Wintertusk, and that is where you get massive xp. You can't get there without completing Grizzleheim first.
It will later on if you complete Dragonspyre too early. You must be level 50+ to move on to Celestia so you will be stuck doing side quests to level up. Take your time, and do those side quests now to level up more.
I started Celestia at level 48. You don't have to be 50.