When can I use regular toothpaste to brush my baby's teeth?

Depends. You can use regular toothpaste at whatever age the child is capable of understanding that they are to rinse the toothpaste out of the mouth rather than swallowing it.
Spitting. When your child can spit on command and you know for sure they won't want to swallow the toothpaste, then you can switch to regular adult toothpaste. (you don't want your baby swallowing a lot of fluoride). Usually by 5 yrs old they know to spit out the toothpaste always.
After they can spit. Great question. I recommend starting regular Fluoride toothpaste once the child can spit effectively, which is around 2 or 2.5 years of age. That way, you ensure they aren't getting too much fluoride, which can be detrimental to tooth development.