"Females under the age of..." instead of "Women under the age of..." Is that offensive?

Is it offensive to refer to women as "females", as I've seen posted at the entrance of a night club:

"Females under the age of twenty-one will have to...."

Although OED traces the usage of female as a noun even further back than its usage as an adjective, the adjectival usage has long been more prevalent. Partly for this reason, referring to a woman as a female tends to focus attention on just a single attribute, one that she shares with many other animals and even plants, at the expense of other attributes that she shares with other (male) members of her own species, such as the capacity for articulate speech.

Thus using a single attribute as a metonym or synecdoche for a person is inherently depersonalizing, so yes I would avoid it as likely to cause offense. Examples vary from the club sign’s calling women “females,” to a waiter’s referring to a diner, or a surgeon’s referring to a patient, by the name of the entrée or operation ordered.

No, it's not offensive at all to refer to women as females. Female is just a little more formal or clinical.