Why must I have my cholesterol checked if I'm a young woman?

All by age 20. It's recommended that everyone have a screen by age 20; sometimes genetics can predispose you to high cholesterol and premature heart disease which could possibly be prevented.
AvpidArterialDisease. It have been known, for over 7 decades, that artery disease typically begins ~age 7, yet remains asymptomatic for decades; artery walls thicken & enlarge, not narrow. Events, including stenosis (narrowing), result from plaque rupture & clots. Optimize NMR particle test (hdl & LDL concentration, not cholesterol), keep hba1c low, optional ?5.0%, sbp ?120 mmhg, don’t smoke, exercise, avoid sugar, etc.