Instead, it's usually both mixed-age and arranged: effectively, families coercing their daughters to join the harem of a rich man. (There may also be a dowry, in which case the family is selling their daughter into the harem of a rich man.)
This is a pretty bad thing, but it's hard to write a fully-general (e.g. gender-neutral) legal definition of it that doesn't also cause problems for regular poly people.
Polygamy is accepted by many religions, and we have no problem trouncing those rights, yet we will bend over backwards for other questionable 'rights' (like the Burqa/Niqab, which is not 'required' except by extremist sects - and is nearly universally regarded as a tool of oppression).
I mean, every everyone is a consenting adult, what does it matter?