I think I found the perfect bikes for each age.

Age 2-4 Strider walking bike

Age 3-4 12" wheel pedal bike

Age 4-5 14" wheel pedal bike

Age 5-6 16" wheel pedal bike ( aluminum frame for lightness )

Age 6-8 rigid 20" wheel mountain bike

Age 8-10 suspension 20" wheel mountain bike

Age 10+ 24" wheel mountain bike

I like micro mini BMX bikes for kids 6-8 year old triathlons. Otherwise the above is perfect