The only age requirement for enrolling children in dance class is that they be toilet trained -- and even that isn’t a set rule. There are dance studios that have classes for toddlers. Typically, classes are divided by age, so for example, a large studio might have different classes for age 4, 5, 6, 7, 8, 9, 10 and 11. In many studios, there may be a lower age limit for teens to be allowed into advanced or performing groups -- this is typically 13 or 14.
There is no upper age limit for social dance lessons, especially in the ballroom world. However, for certain more rigorous styles of dance, such as ballet and contemporary, most people don’t come in as adult beginners. That being said, many adults pick up hip-hop, jazz, Latin, or other dance for exercise and social purposes.