When do babies regulate their body temperature?

All the time: Ortherwise they wouldnt survive long. The issue is they have a very large surface area which makes it harder for them to regulate temperature and easier for their natural mechanisms to fail and thus more prone to hypothermia

Within the first hrs: Studies decades ago showed the newborns ability to establish and self regulate body temperature within the first hours of birth. Initial separation from the womb encounters a colder environment, but babies metabolism kicks in (and we wrap them up). Beyond that first few hours an abnormal body temperature means problems.

From start: You may be referring to the fact that babies get hold/cold more easily than adults. There are a number of complicated reasons for this but babies are able to sweat and shiver right from the start. How effective they will be at temp regulation will depend on level of development because a lot of our own temperature regulation depends on behavioural things like adjusting clothing/blankets.