Actually you can build your own GBs in BA(Delphi, Zeus, Babel and Observatory), what you cannot do is contribute to other players GBsYou are locked from attacking other players until IA.
You cant participate in events or add friends until halfway through BA.
You cant join a guild or open your tavern until the end of BA.
You cant build GB's, do daily quests, or do GVG until IA.
You cant start any settlments or try out reconstruction mode until the end of IA.
You cant do the antiques dealer or pvp game until EMA.
You cant do aztecs settlement until CA.
You cant get special resources/unlock the harbor until AF.
Very silly "assumptions" aka facts indeed.
You can also get blueprints for other GBs if the person you are motivating or polishing is of a higher Age.