If I'm not wrong, it takes 20 years to revert the damage of smoking.
I don't think you need to care at 18, but the effects will be seen over the course of 20 years
Notably risk goes up with the time spent with bad numbers, so the most leverage you have on affecting your lifetime risk is caring as early as possible.
It's an area under the curve situation. Waiting until you start experiencing symptoms is putting a band-aid on an open wound compared to avoiding injury at all by focusing on high impact habits related to diet and exercise.
Focus on adopting the following habits as early as possible: https://www.barbellmedicine.com/blog/where-should-my-priorit...