To bath or not to bath?

Toddler has always loved bathtime
 

Since Toddler was a tiny newborn, we have shared baths together.  When his Daddy lived at home, we would take it in turns to have a bath with him.  Now of course, it's just me.  Toddler still has a daily dip in the bath as part of his bedtime routine but about once a week (usually at the weekend) we jump in together.  Toddler loves it and I often use it as a bribe if he doesn't want to go to bed. 

The only thing I don't like about shared baths is that I like my baths nice and hot, Toddler likes his cold.  I'm not talking warm, but proper cold.  We compromise in that I have a soak in the bath by myself for 10 minutes then put the cold water in at one end for him. 

Toddler loves to pour water over my head, use my legs as a slide and stick foam letters onto my tummy to write his name.  It is such a lovely experience and always very chilled out.  A nice calming way to end the day - no stress, just lots of cuddles and laughs.

But when do shared baths become inappropriate?  Toddler is now 3 and of course he knows that boys have 'dinky's' and girls have bottoms.  He knows that boobies are for feeding milk to babies.  He often compares our bodies when we bath together which in my opinion, is a healthy way for him to learn about the differences in boys and girls.  At what point though is he 'too old' to have a bath with his mummy?  Now at 3?  4?  5?