Is there a more informal or slang alternative to "couple" for two youths in a relationship?

There are lots of slang terms for a romantic couple, or for the state of being in such a couple, or for the actions that make up being in one.

“They’re an item,” yes, absolutely, that’s something people say of the two people in a couple. Each of them could be a “lovebird,” though that refers to the people and not the relationship. “I ship them,” is also a thing, though usually reserved for fictional characters—and also usually applied to more hypothetical couples, two people you believe should be a couple but aren’t actually (yet?). That can be varied to say something like “they’re my favorite ship,” to refer to the (possibly imagined) relationship. There are other words you could use.

But for your example sentence? Couple itself is, in my experience, the most natural and appropriate word choice for that sentence. I honestly can’t imagine ever referring to the other couples as anything other than “couples” in that context. I am a native English speaker from the northeast US in my 30s; I have also spent time on the West Coast of the US. I note comments by PLL and Konrad Rudolph with the same sentiments and experience with Britain.

Beyond that, I have certainly never heard “item” used in the plural to refer to multiple couples; “a few other items around their age,” to me, screams “older author misusing slang from a younger generation.” It just sounds wrong, like one just looked up slang for “couple” and inserted it into the sentence. “Ship” is even worse in that sentence, since people actually out on a date hardly meet the definition of a “ship” in the first place.

Ultimately, I can’t prove or even really provide solid evidence for my position that “couple” is pretty much the only word that fits in that sentence; I have only my experience, and comments suggesting similar experiences, to offer.

item

Slang (in American English) a couple identified publicly as sweethearts or lovers

• John and Joan are an item (Collins)

However, for your sentence lovebirds might work better:

They walked into the restaurant and saw a few other lovebirds around their age.

lovebirds (plural, informal) : people who are lovers : people who are in a romantic relationship

• The two lovebirds were spotted … boating on the water during the day, before hitting the club at night. — Maeve McDermott (M-W)

pairs

To me, "pairs" is more casual for groups of two, while "couples" implies a stronger relationship between the two individuals. From the dating angle, beginning a relationship is sometimes described as "pairing off".

To expand OP's example a little

They walked into the restaurant and saw a few other pairs around their age as well as larger groups of friends

English Club defines young couple as

two young people in a romantic relationship

For example

• The park was full of young couples sitting together or walking hand in hand.