My boys always struggle with 'physical' things. The other lads in the street just got on their bikes and rode them aged 4-5.
DS1 was 6.5 - he was nervous of trying, and we had to bribe him! We did half an hour a day during a half term holiday, and he had it cracked by the end of the week.
DS2 is nearly 6.5 and he can't do it yet, and doesn't want to try.