Based on my knowledge, you are in an era as long as you do not "unlock" further than the advanced scouts research with that colour.
In the First Era the quests start out instructional and then close to the advanced scouts start becoming looped. If you do not have the ones below you just haven't gotten to them yet. You might be able to decline your way until you reach this loop. Declining one sends you to the next, looping the same 9 quests forever and ever.
The ressource-quests depend on your boosted ressource. Each player will have the ressource north of them, southwest of them and southeast of them available for boosting through research.
In my case marble/silk/gems is boosted.
First Era - Advanced Scouts (0/40)
* Train 100 soldiers = 15000 coins, 500 supplies
* Gain 80 marble = 20 steel, 20 planks
* Gain 40 planks = 6000 gold, 600 supplies
* Solve 3 encounters = 2000 coins, 900 supplies
* Gain 2000 coins = 1000 supplies
* Complete a province = 10000 coins, 500 supplies
* Gain 40 steel = 6000 gold, 600 supplies
* Upgrade two buildings to level 4 or above = 1000 coins, 1000 supplies
* Gain 500 supplies = 10000 coins
Second Era - Advanced Scouts (0/60)
* Train 200 units = 40000 coins, 500 supplies
* Gain 2000 supplies = 45000 coins
* Gain 50000 coins = 4500 supplies
* Upgrade 2 manufactories to level 5 or above = 1000 coins, 3500 supplies
* Make 3 trade offers, Gain 200 silk = 10000 coins, 300 supplies
* Complete a province = 4000 coins, 4000 supplies
* Upgrade 2 boosted manufactories to level 5 or above = 9000 coins, 2500 supplies
* Solve 4 encounters = 4500 supplies
* Gain 5 relics = 8000 coins, 3700 supplies
* Make three 24-hour marble products = 200 marble, 2000 coins
* Fight three encounters (and win) = 27000 coins, 1800 supplies
* Make one 24-hour silk product = 100 silk, 2000 coins
* Complete a main hall upgrade = 30000 coins, 1500 supplies
* Gain 100 scrolls, 100 silk, 100 crystals = 10000 coins, 3500 supplies
* Gain 500 silk = 50 crystal, 50 scrolls
* Gain 600 goods = 30000 coins, 1500 supplies
* Upgrade 2 workshops to level 5 or above = 5000 coins, 4000 supplies
* Upgrade 3 residences to level 5 or above = 30000 coins, 1500 supplies
- Two additional quests will be added late in the era, unlocked by the quests you cannot decline.
Third Era - Advanced Scouts (0/70)
* Train 300 units = 42000 coins, 3600 supplies
* Gain 40000 coin = 8000 supplies
* Gain 1000 goods = 60000 coins, 1500 supplies
* Make three 24-hour silk products = 200 silk, 20000 coins
* Make three 24-hour marble products = 500 marble, 10000 coins
* Upgrade 3 buildings to lvl 6 or higher = 25000 coins, 5000 supplies
* Complete a province = 43000 coins, 2300 supplies
* Gain 4000 supplies = 75000 coins
* Upgrade 2 workshops to lvl 6 or higher = 11000 coin, 5000 supplies
Fourth Era - Advanced Scouts (0/80)
- don't know