How often do they change the contents of the museums and is it good for an 8 year old? Last time I took my daughter on Feb 11th,2018 it was filled mostly with toddler things

They change one station in about 8-12 month. For 8 year old, innovators lab, clay and art station, and wooden blocks at basement. But other than that not much.

On average, what are the optimum age ranges for kids to benefit from a visit to the museum?

My kid is only two but I see kids as old as 10 there all the time. I think your question is probably more suitable to be answered by the staff at the museum I'd give them a call and ask about the changing of content.