You use ‘de’ to indicate a property someone has, while ‘el’ takes something from a group. The latter can usually be translated as ‘out of’, whereas the former cannot: ‘Out of (all) my children, she is the youngest.’
If malagxa is not very clear, then malplej agxa is not clear either. Can't have it both ways. But according to the dictionary, agxa means 'old, aged', so no problem there. Still, why not say la plej juna?