So it has been for ages ... just means it has been like this for a long (continuous) period of time. So it has been for ages past ... means it has been like this for different ages, or times, in the past. For example, the Egyptian, Greek and Roman civilisations.

Both are correct, although "in" works better than "for". Even better would be: So has it been in ages past, so is it now.

I don't know the context of this, but the style seems to be slightly poetic and slightly archaic, even biblical. In everyday English today we would say "so it has been for ages" or more likely "it has always been like this". But your quote aims for a more formal style. I hope that helps and makes sense based on the source of your quote.