we had our baby boy in there since 2 days old. he loves it
We used a water wrap I bought off etsy to hold him safely and showered with him from 4 weeks. He hated baths but was great in the shower. That's how we got him clean until he could sit on his own.
mu husband has always taken ours into the shower with him, generally around 6-8 wks :)
I took my son into the bath, but was worried the shower spray would irritate his face and make him cry, plus the worry about him getting slippery.
i took mine babies in the shower from about 4 weeks old, i lay a face washer on my arm and it makes the baby grip better when they get slippery from the soap.
I personally never took my boys in the shower until they were at least 6 months old. For the simple fact that babies are slippery, and this way they can sit up and play. I was afraid to drop my boys or the soap I was using would irritate their skin and eyes.