My daughter is almost 6 and i still bathe with her, but i am her mom. I bathed with my mom until atleast 1st grade. My mom and i still have no qualms about being nude in front of each other. My mom said i used to bathe with her and my dad until i was about 1. There comes a natural curiosity about body parts and whatnot at a certain age. My husband quit being nude around her when she was about 2, then he started wearing underwear in front of her. He stopped wearing underwear only the day she went up to him and patted his crotch and said "Oh no! Daddy pottied in his panties!" because he has a huge bulge!

Personally if i found out in my later years my dad bathed with me until i was school aged, i would be embarassed about it as a daughter. For the same reason picturing your parents have sex creeps you out- the thought of remembering what your dads penis looked like floating in the bathtub might just give you a strange feeling. Keep in mind this is from a womans perspective. I think there is more allowed between mothers and daughters and fathers and sons. Its when you cross the genders that people start to worry about the emotional/sexual development.

Im with whoever said ask your pediatrician.

Just like there are mothers who want to breastfeed their toddlers until 4 or 5 years old, they arent ashamed of it, but most people do find that breastfeeding does have a cut off point (like before your kid can scratch your boob with his facial hair)