how old is too young for tummy time ?

oh ok i just wanted to check because my baby girl is holding her head up by herself already & i want to start her with tummie time

My cousin had her baby do tummy time starting after birth..baby on chest ay first then on the boppy and then without the boppy...

I started on day one. at 1st is was just on my chest then on the floor few minutes at a time a few times a day.