If I didnt bathe with my two lil ones, none of the three of us would EVER get clean! I have a household with ages: 19, 13, 2 and five months. Plus I woh ft. If I couldnt put dd in the tub then grab the baby and jump in myself, I swear to you that we would all be stinky because I seriously would never have time any other way. With my 13yr old, we bathed togather until he was three or four, but that was because he got big and we wouldnt both fit anymore (I have a bigger tub now!).
I quit letting him see my naked when he started not wanting me to see him (shutting his bedroom door, not letting me in the bathroom etc) I figured that was the natural time for it. Also, now that he's almost 14, trust me, he wouldnt dream of walking in on me in the bathroom (though a year ago, he'd plop right down on the side of the tub and talk to me while I was on the toliet, Im so glad that has stopped!
My mom, brother and I saw each other naked all the time and I dont remember a specific time that stopped (never, with my mom), it was a nonissue.
I was molested as a child myself, but not by my mother or brother, and I never thought a thing of seeing them naked/them seeing me before or after because it was so not sexual.
Funny story: DD (2yrold) was in the bath with me and her five month old brother and said "wee wee" pointing to his penis, I said, yes, thats a BOY wee wee, thinking she might be confused because it was diffrent than hers. She simply looked at me like I was dense and said, "no, thats TODDS wee wee". Well, yeah.