Question: Is there a predominant pattern of usage of the terms *girls* and *guys* based on age of the described individuals? Are the patterns the same in the US and UK?

At about what age is it considered inappropriate to refer to young women as *girls*? Is there a point at which such terms are definitely offensive. 

With young men is the issue the same, or can males of all age can fall under the category of *guys*? 

Finally, is *guys* okay to be used in reference to women when not contrasting them with males, or is it considered inappropriate?

Answer: 
The most common complements for the terms you mention are 

> 
> * male/female
> * boys/girls
> * guys/gals
> 
> 
> 

Usage is far from parallel. While arguably *boys* and *girls* should be reserved for underage (whatever that means) people of the respective genders, adults, especially younger adults (probably under 35) often refer to their own gender by the term of children of that sex.

However, refering to an adult using childish terms may be considered demeaning. It is generally more acceptable for men to refer to themselves and their agemates as *boys* and vice versa. When the child referents are used by the opposite sex, it is often seem as sarcastic or even insulting. Many adults will assiduously avoid referring to the oposite sex as a *boy* or a *girl*.

This is even more of an issue when dealing with groups that may have been forced into lower social status based on race or socioeconomic status. Do **not** refer to an African-American male as *boy*. This is highly insulting. To a lesser degree, an adult male referring to any other adult male, except for close friends as *boy* would be considered demeaning. The same rules generally apply to women (although possibly to a slightly lesser degree).

In some circles, even older women may refer to each other as *girls* (perhaps with a slight degree of irony), but such reference by a man may well invoce umbrage. 

Finally, the use of the terms *boys* or *girls* for individuals over 70 may occur, but in an ironic fashion. Whether that is welcome depends on both the speaker and the audience.

*Guy* and *gal* are informal terms, much more polular in a period that ended in the early to mid 2oth century. They are still used colloquially, but often with a but of irony.

Many people use the term *guys* to collectively refer to a familiar group, regardless of gender. This term seems to be used somewhat more by men than women when refering to women or a mixed group of men and women. In general, most women do not seem to object to the term when used in this collective way. 

The singular *guy* is generally applied only to men. To suggest a particular woman was a *good guy* would seem a bit off, regadless of who used it.

All ofthe above refers to US usage.
It depends entirely on what you are writing and who you are writing for.

There are no hard and fast conventions here in Australia, which is more or less the same as the UK when it comes to English usage.

To demonstrate the importance of the writing type and the audience, let's consider you were writing an article in a newsletter for an aged-care home, and were part of the community. If the article was an informal account of an organised bus tour where the female residents went out to lunch and then saw a movie, you could refer to the women as girls with impunity, as that's probably how they refer to each other.

On the other hand, it'd be weird to do so if you were an outsider writing about the female residents preparing a community dinner.

I think the best rule of thumb is to consider which terms a representative sample of your readers would use in that context. And then use those terms.

Don't forget, there are also 'young women' and 'young men' 'older women' and 'older men' 'gentlemen' and 'ladies'. And probably a bunch more.

I think 'guys' as a mixed gender collective term should be avoided unless it's used to address a mixed gender group of people in an informal manner, for example at the start of an email or speech.

'Hi guys'