I'm 14, but started seriously biking when I was 12.
11 years old-Started biking around 10 to 20 miles, just for fun
12 years old-Was bikingup to 55 miles
13 years old-Biked my first solo century and started "training"
14 years old-Am now training seriously (Intervals, HRM,etc.)

My two friends that are also my age and have similiar achievements, now have their 10 or 11 year old brothers on road bikes. I have tried several previous hobbies, but I know that biking will be the one that sticks with me the longest. I know I don't hjave experience, but I don't buy the "burn-out" theory. As long as you're having fun I don't see how you could "burn-out". Wouldn't getting serious into biking at a young age (Not too young!) make for a better lifetime of biking?