You have to have every 10 levels of every tech to advance into the new era/age. For example: You need every tech to be at level 10 to be able to reach Iron Age. You cannot go past the level cap of the era, so remember to keep researching the techs you think you need right now. So before you enter Iron Age, you cannot go beyond tech level 10. You gain 1 science point per province (this can be upgraded by buildings). Tech requirements for resources are (remember that wood can be gained from forests and horses can be gained from plains and grasslands, see climate map for these things)