T.D.
My daughter is 6 and my son is 3 and they are still bathing together. They have not expressed interest in bathing separately, nor am I really considering separating them anytime soon. They are not rambunctious and they entertain each other. They know that the other one has different body parts, so they pay ZERO attention to those body parts.
Unless they have a problem with it or I feel it's getting awkward, I will probably let them bathe together, at least until my son is old enough to shower by himself.