How old should baby be to start doing tummy time? When we had an appointment on Tuesday his doctor…

How old should baby be to start doing tummy time? When we had an appointment on Tuesday his doctor asked if I was doing a little tummy time. I said no not really. She said she asked cause when she flipped him on his belly he could lift his head so well. I said he was born like that. Lol. He has been doing that since he came out. But she told me I could do it for about 5-10 mins as long as I keep an eye on him.