2-3 years. Most dance studios have classes for toddlers, but expect them to be more exploration of movement than dance training. Most two and three year olds are wiggly and have short attention spans! It's also a common age for separation anxiety and you may not be able to pry them off your leg long enough for them to participate.
Five years. You and your baby can start gentle movements to singing or music right after birth. You can encourage motor activity to music at any age. By age 2-3 years, creative play "dance" may be encouraged. Chronological and developmental age are not necessarily the same. Some children may be ready for classes at 4, some not until 7. Nevertheless, it should always be fun. Encourage, but don't force it.
3-5 years. Some children are ready by three years. Others not until later. Some dance classes do not enroll until after five. I agree that when your child is ready and willing is the time to consider enrolling in a class that meets your needs and your child's needs. Having fun while learning is very important.