The two little ones have a kitten proof room with all the amenities. For now they get a couple hours "house time" during the day and a couple more at night. The 9 month old is a surprisingly nice house kitty. They all respect the 15 year old.
Watch for a true Hunnic Reign of Total Terror when the 5-month-olds get to be about 9 months old. As you already know . . .
Don't be afraid to crate them (in a nice roomy cage) till they're old enough to act right. Unless you enjoy them playing Sabre Tooth Tiger with your toes in bed at night. On top of all else. At least they'll be too busy with each other to bother the elder much.