"They are younger than me."
Translation:Maen nhw'n ifancach na fi.
No, ifanc is young, not 'younger'
iau is 'junior', or 'younger', but 'ifancach' (or its close variants not shown here) is probably more common.
A problem with iau is that it can be confused with its other usages and meanings - Iau (Jupiter), dydd Iau, and also 'yoke' and 'liver'!