Best kids bikes for each age

There is an assortment of bicycles for adults such as road bike, time trial bike, fixed gear, mountain bike, triathlon bike, BMX, Fat tire, twenty niner, full suspension, hybrid, and many more. Our lead designer, Dr. Yan, will summarize experiences on which bikes are best for different applications.

Ages 1-3
Balance bikes are perfect for this age. Strider has the shortest frames, hence they are best for the youngest kids. However any balance bike is perfect for bigger kids.
Age 3-4
This is the perfect time to transfer from a balance bike ( 12" wheels ) to a 12" wheel pedal bike. Don't push your kids to pedal, some will do it right away others will prefer to keep the non-pedal balance bikes. Let your child pick their preference. My daughter started a pedal bike at a very young age, while my son waited a year longer. Both situations worked out perfectly.
Age 4-5.
Kids finally get to ride a real bike.
I got my daughter a BMX micro mini at the age of 4. Micro mini bikes are super lightweight ( about 15 lbs ), and easy to adjust for different heights. My daughter used her micro mini for about three years. She also used it in a few triathlon races for kids ( kid's tri's are a blast! )
I got my son a 16" BMX bike when he was 4. It was short and fit his body perfectly, however the standard wheels/tires were very heavy and he had a hard time climbing hills until he got older.
Overall I think 16" BMX or micro mini bikes are perfect for kids up until 5.
Age 6-8
Small kids still need light bikes, so please stay away from department store models that use hi-ten steel frames. Once they outgrow a BMX micro mini, a BMX mini will be slightly taller and longer and still super light ( 15lbs ) so these are ideal for keeping up with Mom or Dad. They will also be able to use these bikes in triathlons.
Kids interested in BMX can step up to normal BMX bikes for jumps. would be a good resource to find an BMX bikes.
I shopped for mountain bikes and I was humbled there is not much in the market. I will address each manufacturer and describe pros and cons. Eventually I ended up designing my own Downtube Race mountain bikes, the rigid model is perfect for smaller lighter kids, and the suspension model will stay with them until they are 11 or so.
Performance has some of the heaviest kids mountain bikes, freewheel drivetrains are another minus, and the fork has to be the lowest quality model in production today. I believe this is the worst kids mountain bike in the market.
This is the top selling brand in the USA. I was shocked that their 20" wheel offering had a threaded headset, 7sp freewheel, and non-replaceable chainring for $600. FYI all of these are unacceptable for a quality bike. 
They actually have a nice kids bike on their website however it was unclear if they used a steel or aluminum rigid fork. I was unable to find one locally, so I still don't know. However I suspect it is steel.
Ideally a kids mountain bike should either have suspension for comfort ( older kids will love it ) or a rigid aluminum fork ( younger kids require lighter bikes ).
The only bike with a 6sp freewheel drivetrain ( I didn't even know they made this stuff anymore ). Low end crank and the cheapest/heaviest seatpost saddle interface.
My kids were not getting on this bike :)
They go with a hi tensile steel fork, threaded headset, 7sp freewheel drivetrain, and low end crank. At this point I was worried that I may never find anything for my kids.
We developed our kids mountain bike because there was nothing good out there. We include a 9sp cassette drivetrain, replaceable chainring, adjustable angle stem, suspension or a rigid aluminum fork ( both have aluminum lowers ). Additionally our bike is disc ready with disc hubs, and tabs on the frame and fork. We developed the bike to improve on all our competitors faults, additionally we sell it for less than most of the competition.
8-10 years old
My kids outgrew their 16" wheel BMX bike around this age.
Triathlon racing: The best option is a BMX Cruiser mini ( with 24" wheels ) at around 16lbs for a one speed.
Road bikes with 650C wheels and 46-48cm frames may fit your child, but make sure you use a single Chainrings up front....the double is always a problem for my kids.
Mountain bikes: The bikes in the 5-7 age range work for 8-10 year olds. However bigger kids prefer suspension.
Age 10 and up
Currently I am not an expert at this range, however I understand standard BMX bikes are good, 650C road/tri bikes with small frames should fit, and 24" wheel mountain bikes are good options.