Civilization – What amount is needed to construct the World Congress projects

The amount is unlimited. You can have level 15 warriors in the ancient era, in theory.

You might have been confused by the fact that a unit with 30 XP or more cannot gain any XP from barbarians... but this is independent of era.

World age affects the prevalence of hills and mountains: younger worlds have larger, more craggy mountains that appear in bigger clusters whereas older worlds have more sloping hills that tend to appear in smaller clumps.

The code that governs this is in ContinentsPlus.lua:

local world_age_old = 2;
local world_age_normal = 3;
local world_age_new = 5;

-- Set values for hills and mountains according to World Age chosen by user.
local adjustment = world_age_normal;
if world_age == 3 then -- 5 Billion Years
  adjustment = world_age_old;
  adjust_plates = adjust_plates * 0.75;
elseif world_age == 1 then -- 3 Billion Years
  adjustment = world_age_new;
  adjust_plates = adjust_plates * 1.5;
else -- 4 Billion Years
end
-- Apply adjustment to hills and peaks settings.
local hillsBottom1 = 28 - adjustment;
local hillsTop1 = 28 + adjustment;
local hillsBottom2 = 72 - adjustment;
local hillsTop2 = 72 + adjustment;
local hillsClumps = 1 + adjustment;
local hillsNearMountains = 91 - (adjustment * 2) - extra_mountains;
local mountains = 97 - adjustment - extra_mountains;