Many will have married already, so choices when I was in my 30's tended to be:

• Somebody who didn't marry because they really were careful about who they want to date or settle down with. This could be fine, but could also mean unrealistic expectations or some issue that hadn't worked through previously (I was in the latter boat)
• Somebody who married and divorced, possibly due to a relationship that went bad. They may be the cause of that, the victim of it, or other circumstances. Increased chance of kids and ex issues
• Married but the spouse passed away (30+ is increased chances for previously unknown medical issues to crop up, it just a car accident)
• Somebody who didn't have it together enough (in their mind) to even consider relationships. Could have been persuing a career or took a bit to get past the "wild party" stage
• Immigrants. People who worked to get out of one country and to another and relationships weren't in the cards yet. There can be possible language/cultural miscommunication hurdles (including different social cues)

I wouldn't say it's the dregs. Just that one's choices have changed a lot from "the one a couple houses/lockers down from you".