They do always seem to amaze. My two sons are being encouraged to ride. The first ditched the stabilisers when about 5 but the second was much sooner, just before turning 4, as he had big bro to catch.
Son #1 did the approx 10 mile lap of Derwent and Howden Res from Derwent dam (dam busters practice site) at about the age of 5½ on his Ridgeback MX14 which was starting to look a bit small as he approached 6 so I rushed out and bought him a GT Stomper 20. Both proper bikes, not the usual toy shop BSO's most kids get but well worth the money and #2 is now enjoying the MX14.
My oldest son is now nearly 8 and I am beginning to worry about the day he can ride harder, faster and further than me. Having said that, I am looking forward to some epic MTB advetures with them both.