Toxins love to get you while you're young. Lead mercury secondhand smoke and sundry other environmental nasties do a lot more damage when tissue is immature vulnerable and growing than when it's mature and comparatively fixed.
I remember when I was 6 years old and my brother used to go seek out guys that were 13 to come over and play football against me while he was the 'permanent quarterback.' I didn't know exactly what the age difference was but I was already playing against older guys.
