I agree with Trunks, the question of 'When to advance to heroic' depends on your economy, when your enemy advances, and your personal preference (in that order of importance). Going Heroic without a strong economy will gain you only a small advantage, as you wont be able to afford many of the upgrades that Heroic offers. Those that do go for a fast heroic often do so for the economic upgrades (if focusing strongly on economy at the expense of their army) or siege weapons (to be able to quickly take out buildings before their enemy can). Also for certain godpowers they have a plan to use effectively (like Ancestors/Eclipse). Going heroic too fast leaves you vulnerable to attack however, as your army will be small in order to save resources.
Staying on-par agewise with your opponent is important so that your army stays balanced with theirs in the amount of upgrades they have. This comes into play mostly when both opponents are agressive and/or evenly matched, otherwise the more agressive opponent (with a larger army) could still destroy their opponent who went for a fast heroic simply by having more troops (even if their opponents army is better upgraded). In general, your number of troops is more important than the number of upgrades, especially in classical age where few upgrades are available and army production is just getting started. Of course, how well your army counters the enemy army plays a large part in who is successful in battle as well as the number of troops, as a smaller army could defeat a larger army if it counters it well enough. This means that if your army counters their enemy army well, and you expect to be better able to counter their army than vice-versa, you can better afford to slow military production and go heroic sooner.
Lastly, personal preference (including your main and lesser god choices) will effect when you advance. For instance, if you enjoy playing agressively, you are likely to advance later than average due to a higher rate of military production in classical. If you like having fast units in your agressive strategy, you will likely be spending large amounts of food on your army, further slowing age advancement. If you enjoy playing defensively, then you may advance more quickly than average (but not necissarily). Some people play defensively in order to advance more quickly by having little to no military unit production, instead keeping battles under their tower fire and making sure their military counters the enemies well (hopefully with a ranged, non-food-costing unit). Other people who play defensively (that is, produce less military) do so to expand their territory with buildings, at the cost of a later age advancement (but maybe still earlier than an agressive player). Mostly towers and TC's are what they build, to increase their tower fire advantage, claim more land as their own, and increase their civilian unit production. Also, if they take a TC near their enemy's base, they are in a good position to get 4 TC's later in the game and have a population advantage.
What gods you pick can help a fast advance as well, as any ecomomic advantage can be used to advance more quickly. For example, Odin (and to a lesser degree, Thor) have advantages at hunting which can be used to advance faster. Or Ra, if he invests in farming during classical, can use his godpower to help advance quicker. Of course, gold is needed to advance as well, which Isis/Thor has an advantage at gathering. Even Gaia, with inreased wood gathering from her forests, can use this advantage to increase food/gold production by tasking fewer villagers than normal to wood.
Anyway, I'm sorry this is so long, I can be pretty longwinded sometimes. I still didn't even write everything I wanted to, but I hope this helps answer your question. In short - There is no easy answer to your question.
[This message has been edited by Mune84 (edited 09-17-2006 @ 05:32 PM).]