At what stage should i start to develop large games?

at what stage should i start to develop large games?
plus my whole team is made from levels 5 and when i search for a new worker all i get is max level 2
does it worth to hire them and train them?
or otherwise how can i find higher levels workers?

At the stage when game is telling you so. So in moment when you have enough fans (100k+) and have at least 4 employee.

I usually hold off on large games until the 4th office with 6 employees, it helps ensure nobody is overworked. There are 7 maximum workers, but 9 areas of development. Decide early on which genre you want to specialize in. Action, simulation, RPG, and adventure all only put importance on 6 of the 9 areas. Assign one employee each to the important areas, assign the seventh to all three unimportant areas.

what are “areas of development”?

Engine, Gameplay, Story/Quests, Dialogue, Level Design, AI, World Design, Graphics, and Sound.

one more question, my entire team is made of levels 5, while when i hire a new member the best i get is level 2.
is it a bug or am i doing something wrong?

This is normal beheavior. A good specialist is not allways available, it is like reality.
You need to search again, a bit more investment maybe help.

Depends on how much you are spending on finding new staff.
But in general it’s better to have low level new staff, since they still have room for improving.