on a track? around 10 you'll be the right size for it. maybe earlier if you've been racing for a while and can deal with a bike too big for you.
on the street, 16-18 depending on where you are because you need a license
'14 grom, '03 sv1000 x2, '03 sv650 trackbike, '08 wr250r, '0something crf100 mini supermoto
Previously: '05 gs500f, '07 sv650, '09 dorsoduro, '98 yzf600r, '04 sv650, '06 sprint st, '95 rm125 supermoto, '04 ktm65 mini supermoto, '72 cb350, '01 super sherpa, '04 nsr50