Hi,

When my DD was 18 months old i took her to a local run gym every friday morning The class was for 18 months-3 years old. She was the youngest. We did 2 terms. I didnt feel she got much out of it. The classes were very structured, which is great for a 3 -4 year old, but an 18 month old is not going to sit and listen to instructions during a 45 min class - thats what i found anyway. The activites they did do each week, i could (and did) quite easily do at home.
I also found it expensive. A push on the swings at the local park, along with some ball throwing, followed by a baby cappucino or strawberry smoothie afterwards was much more enjoyable - for both of us.