One of my complaints about the age up for all the kids (aka they added 3 years on the ages of everyone, made the robert rebellion happened 3 years earlier, and removed the reign of Jaehaerys ii to make the timeline fit) is how adding 3 years to Joffrey age changes his personality.

There is a difference between a 12/13 year old being a little sh!t. A complete difference when we have a young adult at 15/16.

At the mythical age of 16, Joffrey inherits the Iron Throne, Westeros would no longer be ruled by a Regency of Cersei and Tywin but instead by the young king Joffrey. Tywin couldn't order Joffrey to bed, no instead Joffrey could order the kingsguard to remove Tywin's head.