It completely ruined me.
I have been in or around a theatre since I was 8 (I am now 38...ish). I'm not sure I'd put much significant correlation between those who take drama at a young age and those who pursue it as a career. Far more people take drama as a child and go into some other field than take drama and continue on into their adult years.
For me it helped me in front of audiences and large crowds. It's also pretty invigorating to create something and have people and your peers like it. I didn't have much of that growing up due to the fact I had to wear a gall-dern eye patch until I was 8. Wait..this wasn't a thread to let out all of our deepest, suppressed childhood memories?