Hello and welcome,
This is how I do it. I do not go by age , too many variables like DNA etc. involved . On my turtles once they are 4" in size I skip to every other day or two "Only" on the protein side. But they will get veggies everyday !!! Say a leaf lettuce a piece the size of there shell is good. They should be on 70% or more veggies by this size since there are not a baby growing at a rapid pace. Also when I feed protein I use the head rule or even the 2/3's head rule.
Good read here :
http://www.redearslider.com/pellets.htmlYou may want to feed separate so one does not out compete the other. If small tank to help keep it cleaner longer feed in separate container . Also let them poop there to , normally poop after they eat , helps on maintenance in regular home!
Always better to under feed than over feed to prevent health problems !
A baby and two juveniles , you understand the day is coming for separate tanks .