RE: How early...
I started both of my kids at age 4. My son started with cheap radio shack and tyco stuff 3 years ago, but my daughter started last christmas with an RC18MT. My son turned 7 last year and I got him his first nitro RC, a CEN ATX buggy. I of course bought myself a nitro truck so we could do drive together.
Another question would be how early do you turn them loose with a big block Revo?