aging up a newborn in sims 4

Newborns age into infants.  The sims 4 did not have toddlers or babies unattached to the bassinet when if first came out.  Toddlers came out many  years ago, and infants came out before the Growing together expansion.  If you are new to the game, I will let you know that infants behave differently ( have a lot more things they can learn ) if you have the Growing Together expansion.

Enjoy this new Infant stage, it's a lot of fun, although it can be a bit of work.

Not quite sure what you ask? Do you have problems aging up a newborn to the infant lifestage? I think it can not be done right after the birth, there is some tech stuff there but I think it is by design. I'm not sure if it just need some time, like 3-4 sim hours, or if you need to interact a little with the baby. When I had problem aging it up it was normally possible after 4-5 interactions. 

I have not had any issues aging infants up in this game. I just click on the cradle after both parents have gained a friendship with the infant and select age up.

With toddlers I make birthday cakes and age them up by helping them blow out the candles.