How does baby aspirin help your heart if you are in your 30's or 40's?

Depends: Assuming you tolerate low dose Aspirin and have had no event or multiple risk factors, epidemiologic data would suggest that men may benefit in their 40s, women in their 50s, for the anti-platelet protective effect on anti-atherothrombotic plaque conversion from stable to vulnerable lesions. If, however, you've already had an event (secondary prevention), then Aspirin is useful at any age.