Really depends on the people really and what the employer does to support/encourage graduates. There can also be much older people on "graduate" schemes who've joined the scheme after other jobs/careers.
From my son's experience, most of the "grads" didn't really mix together. It was a huge firm, with dozens of new joiners to their grad scheme each year in that building alone, there also being dozens in several other buildings. I seem to remember him saying there were 300/400 new joiners every year across the entire firm. They were spread across different departments/sections, so the "grads" didn't actually work alongside each other, they were all just attached to very small teams and worked only with the rest of the team, all of whom were more experienced/qualified/older etc. Many of the grads didn't live in the city where they worked, so weren't really interested in nights out etc as they all just scuttled off home at the end of the day, some having as long as 90/120 minute commutes from other towns and cities. Some of the grads had partners/children so likewise weren't interested in socialising. I think he said the oldest "grad" was in her 30's and joined after spending a few years as a teacher to do a complete career change - she was apparently nice and friendly, but had a husband and a couple of children, so not interested in socialising outside work.
The firm themselves did all they could to encourage the grads to mix and socialise, they organised weekly "grads" lunches, monthly "grads" drinks, bi-monthly "grads" meals out, set up "grads" social media groups before they started so they could get to know each other, talk about sharing flats etc., but son says it all fell very flat as the people were so very different. Many would go to the first "event", but never to be seen again.
What he did say was that the "interns" had a much better community, i.e. the current Uni students doing their "year in industry". They were only there for a year, were mostly living in student accommodation or hostels or house shares, and as they were mostly the same age and position in life, they got on more, had more in common, etc. Son joined in with them more than he did with the other grads, and has stayed in contact with many of them, including going to stay with some of them for weekends etc., once they finished their year and went back to Uni.
So, cut a long story short, it depends on the cohort really. If a firm only takes in new grads straight from Uni, they're mostly going to be on the same page, in a similar position in life, and probably "up for" continuing the student lifestyle. However, if the firm take in a broader make up of the cohort, including older grads, career changers, etc., then the grads won't gel as easily.