Second Hand Boys’ Bikes

Indeed, with children's bikes the differences are even less obvious as many machines - aimed at boys and girls - feature a slanted top tube design. Second hand boys' bikes might sometimes be slightly larger in terms of handlebar width, but any differences are minimal in all honesty.
Where second hand boys' bikes and girls' bikes do differ is with the design styles. Gender stereotyping does definitely still exist with children's bikes - just as it does for many toys - so for girls' bikes, expect to see lots of pinks and flowery designs. For used boys' bikes, they might have a bit more of a masculine look with blacks and bold colours, such as dark blue and red commonly found.
You can usually spot a bike that is marketed at boys straightaway purely because of the colours of the design. Essentially though - and structurally - there is very little difference between a second hand girls' bike and a second hand boys' bike.
The key thing to consider (other than design preferences) when buying second hand boys' bikes is to remember that it's all about the wheel size. The size of wheels range from 12" (which would typically suit a boy between the ages of three and five, up to a wheel size of 24" - normally suitable for 8-11 year olds.
Ensuring that the ride is comfortable and therefore safe and stable is the most important thing when buying used boys' bikes. Allowing some flexibility for a bike to be 'grown into' is sensible, but essentially it's the combination of a boy's height and his inside leg measurement that will determine which size of wheel is the most appropriate between the range of 12" and 24".
Girls' and boys' bikes are structurally the same, it is only really in their designs and colours where they differ.