My son got extremely upset with his bottles around 2 months. At that time we were using the level 1 nipples. I switched to the level 2 and he's been fine ever since. I think the milk wasn't coming out fast enough for him with the level 1's. I think the box of the level 2 nipples suggested starting them at 3 months but dI what you are comfortable with. Every baby's different and you know your little one best
I think 3 months sounds about right for when they normally recommend switching to level 2. I will say that if baby is happy, there's no real time table.
Re: dr brown bottles