Is there a specific age or ages that a woman's metabolism slows down?

Not specifically: Everyone has different habits and metabolisms so it's not one size fits all age.

Thirties, generally.: Our metabolism starts to slow in the mid-30s, which is why starting aerobic exercise before then and continuing it through then and beyond is so important for health and well-being and vitality. By the way, women start to lose bone strength in the mid-30s, too, so start supplementing calcium and vitamin D at that age.