Ultimately, if such an idea was implemented, young adult workers, without the right to vote, would have to hold their water till they were old enough. They wouldn't be too neglected by the policy makers because people do vote for their kids too. (Remember, Blair's 'education education education' slogan?)
15-18 year olds, still in our education system, are very much at the mercy of the state, but don't get the right to a vote because they're not deemed ready for the responsibility. I'm just suggesting that people in their early 20's still aren't ready either. If the government treats them poorly enough, I'm sure they would remember, and my well vote accordingly when they are old enough.
I get your theory and do agree with it to a point, would be a bureaucratic nightmare to run it though
I'd keep it at 18 to be honest, at the end of the day you can't legislate for maturity or education so pointless raising it or lowering it. I'd rather focus on changing the voting system rather than the eligibility of who can/can't vote.