One could conceivably argue that the 'Me, me, me' views of the majority of baby boomers shows a distinct lack of societal awareness incompatible with the proper exercise of voting and thus should disqualify that whole generation from voting. One could conceivably argue that parents are unusually influenced by thoughts of issues affecting their children to be expected to vote rationally and should be banned from voting. One could conceivably argue that the failure to vote in an election even once demonstrates a lack of faith and understanding in the electoral process and that they should be subsequently banned from ever voting again.
The voting age has reduced to eighteen because this is the general consensus on when a person is deemed to become an adult and the idea that every adult has the right to determine who represents them in a democracy. The notion of taking away the right to vote of any adult based on a perceived lack of maturity is effectively condemning them to a second class status.