There was a King.
He made Paradise Island what it is until the start of the series.
Somehow he exerts his will onto every Royal who ingests the Founding Titan, so no Founding Titan holder to date has been able to enact a rebellion.
Eren is a commoner who ate the Founding Titan. He can't use its powers unless he touches a Royal.
The king is dead.
Titan user's age is irrelevant. They live for 5 or something years once ingesting a special titan, we're told.
And before anyone asks, Zeke is a Beast Titan holder with Royal blood. If you eat him you inherit the Beat Titan power. You don't inherit his Royal blood.