It is a strangely incompetent webpage from Boots - it has two different minimum ages for purchasing the product.
I think the higher dose in the men's version does create a risk of unwanted growth in other bits of the body, so that might be the reason that particular product is not for women.
As someone with alopecia, I did also just want to mention that there are many different causes of hairloss, and many of them won't be helped by this drug. I'm sure you probably already know that. But there is such an industry of semi-respectable businesses scamming desperate women with missing hair that I wanted just to get it on the thread