My son is 4 & has been riding his cnoc 14 since his third birthday. He is starting to look a little big for the bike now & we were thinking about buying the cnoc 16 but when I was looking through the sizing specs, I realised he is almost big enough for the beinn 20. Should I wait until he fits that bike or should I just get the cnoc 16? I'm concerned that he may struggle with the gears if we get him the beinn. Any thoughts or experiences would be appreciated.