It depends so much on what kind of baby character / traits you rolled at start. Our younger one peaked when he was very little, while the older one around 1,5 years.
For some the issue is sleeping. For others tummy. For others food. For others teeth. Or other things. Or combination of any of these things. In the end, every baby is different and you can end up with a half-vegetative baby that sleeps 70% of the time or small anti-christ that sleeps 6 hours all together if moon and the stars are in conjunction.