interesting idea considering that all 3 are ageless demons doomed to be 12 for their whole lives. .....This raises the question on Mid-boss and Laharl's dad. But other than that, i like this twist :D
Actually, there's no reason to think they don't age at all. IIRC, the ages the three gave were between 1200 and 1400. Which, if you divide by 1000, gives a pretty good estimation of their apparent ages. So, maybe they just age at 1/1000 the human rate.
Great pic, but sadly, i call fail on Flonne's wings. They're supposed to be devil wings. Unless she got heaven's forgiveness in that span of another 1000 years.