Power Rangers ended up retconning the original cast (who were late teens early 20s playing teenagers) from 18 down to 14 when the second season was commissioned in order to not have them leave school. You could tell, there's no way the blue ranger in particular (he was the oldest in real life) could pass for a teenager.
There are many other examples. The BBC are trying to allieviate the problem by filming two series of SJA back to back, so the kids (specifically Luke, since the others are 20-somethings already anyway) don't age as much between series.