What is the term for young adult male/female (aged 18 to 25)?

I think the best you can do here is

A young couple emerged from the shadows.

If you need to specify gender, I would use young man or young woman as appropriate. While I agree with Charles that in spoken English young man is used more towards children, I would posit that this is the case only when addressing them directly. I could, for example, address a 15 year old boy as young man. It would make me sound professorial, but it would not be out of place.

However, if someone were to tell me they

saw a young man entering the house

I would think of a young adult, not a child.

Update:

The closest to guy would be youth but it only applies to males when used to refer to individuals as opposed to groups. Better, but also exclusively male, is lad but that is also more British than American. Kid can be used, but is ambiguous. An older person can refer to young adults as kids but it usually means children. Finally, and probably best in your case, you can use youngster(s). Once more, though, this is a term that is used by older people towards younger ones. It is more used for teenagers and young adults than for actual children but it also carries an implication that the person using the term is older than those she is describing.

"Young man" and "young woman", if you want to call attention to their age.

As Jim said, "young adult" is what I would say. "Young woman"/"young man" somehow seems much younger in casual conversation. It's what people would call me when I was a teenager or pre-teen.

But in literature, you can say "a twenty-something" or "a youngish man" or "someone who looks college-age" or any number of other descriptors. What about their age is relevant to the work?

In work I do with this age group, they are always referred to as youth.

EDIT:

After pondering your request for a word "similar to 'guy'", I have thought of chap and fellow. The disadvantage of both words is that they do not specifically refer to youth. Furthermore, they are in no way gender-neutral -- both refer to a man or a boy. However, to me at least, they carry a connotation of sprightliness that I for one would associate with youthfulness. To be clear, they don't mean that the person is 18 to 25 years old.

(I myself would still go for youth, though)

You could put the setting or make reference to their ages earlier in the story, then readers will know the rough ages of the characters and you may not need to say 'young adult' later on.