My son kind of learned when he was 7 - he managed to go a few yards, and considered himself done. He tried again a few months later, but couldn't do it. He really had no interest in learning, though. What worked for him was a scooter, this past summer (when he turned 8) - at first he was very hesitant on the scooter, too, but he actually enjoyed it, so gained confidence with the balancing. After using the scooter over the summer, going faster and faster, I persuaded him to give the bike another try. Lo and behold, he got on and rode it straight away without a problem
