It varies by manufacturer these days. It used to be 16 and up was "plus", then the fashion industry bumped it down to 14, now I'm hearing 12. It's all bull turds. The fashion industry is doing that to make money. Plus sized clothing costs more to buy. I doubt it costs much more to make; definitely not the extra amount the manufacturer charges. I've seen excise charges of $20-60 more per item just because it's "plus sized" or worse "extended sized".
WR
(and yes, I hate the "fashion" industry, why do you ask?)