I think a tiered minimum wave system would be a huge mistake

Personally, I think the tier system is a mistake.  Let's take an 18-24 year tier as a for instance; it's easy for me to figure out). 

18-24s have double the unemployment in most modern economies.  Say US overall unemployment is 6.5%.  18-24 unemployment would then be about 13%.  A tiered wage might drop that to 8% at best (guessing) -- 5% more youth employed -- while 87% are being paid much less ($10 an hour instead of $15). 

100% of over 24 year olds will spend the rest of their lives looking over their shoulders, worrying that the underage are going to keep them out of a job (which may set up the "sneaky", when nobody's looking, race-to-the-bottom).

Ask the unders; and ask the overs: I suspect they wont think anyone would be doing them any favors.