My daughter and I did a parent-tot music and movement class when she was that age, and it was great. She loved it, it was a good way for her to practice basic social skills, it got us out of the house (I was always looking for things to do out with her because she loved the stimulation, got bored quickly at home), and it led to several connections with other parents and to playdates. We still sing some of the songs we learned there nearly three years later.
The music class was for toddlers, so there wasn't an age issue, but she did outgrow it far before she reached the age for the preschoolers class. You're probably right on track joining this class at the young end of the range.
Also, in the past year we have had great success enrolling my daughter in enrichment classes and camps meant for slightly older children. (The first time I tried it I lied about her age, but I felt so guilty that I haven't done that again!) Instructors I've encountered are surprisingly open to letting her enroll even though she doesn't meet the age requirement. I usually explain that she is advanced/competent/passionate in the class topic and that she is mature and prefers the company of older children so should fit in well. Those have been her happiest group experiences so far.
