Feh, you don't really need extra cubes. Poker chips work great as a substitute. I like the idea of the VP reward for age ups. I'd do it as a one-off lump sum. IE doing the Age up gives you an immediate reward in VP. To encourage people to be first you could give whoever is first extra VP so whoever is first to Classical gets 2 VP instead of 1.
Your reference to poker chips has inspired me to include some in my copy of AoM to function as Next Age tokens.
Red - Classic Age
Blue - Heroic Age
Green - Mythologic Age