What age do you stop letting your kids see you naked?

I am a first time mom to an 18 month old baby boy. Lately he has become very interested in my body, he pulls up my shirt and plays with my belly button but insists on exposing my breasts as well. Sometimes he pokes my nipples or lays his belly against mine. I know it's all innocent at his age, but it just got me thinking, at what age does it become inappropriate? I should mention I'm 11 weeks pregnant and this belly fascination is recent, so maybe he is picking up on that fact?