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
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
-- 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;