I don't know if Steven is 11 or 12 years old but if they kept him in that age, it would cause big problem because his voice actor is 18 years old and too old to play little boy.
I know Steven's voice got deeper but at least, made him little older (NOT in Grandpa's Age! Not Uncle Grandpa one! The Normal GRANDPA'S AGE!)
Remember, Steven is Half-Human, not completely fully immortal like Gems
It like having a 80 years old actor playing 10 years old boy! Who done that?!