Don't think there is a right answer when it comes to adults accompanying visits - but its definitely worth talking about.
When I was going to uni my parents didnt show any interest at all, I did everything on my own, which was kind of normal back then, including really long train journeys and stuff.
I have always encouraged my young adults to go to Open Days and Offer Holder days on their own, invariably they have declined. Partly for logistical reasons and partly because we talk stuff through as a family so its kind of normal for us.
I have also encouraged them to try and go with friends but that hasn't worked out in the end, partly because the other families wanted to go with their young adult.
As others have said, some Unis are really good at separating adult and applicant, which I thought was helpful.
Because much of Open Day and Offer Holder day stuff is curated by the uni I also encourage mine to wander off on their own, think about the people they met (student and applicant) because these were potentially their future friends. Its risky because you only meet a small sample, but it definitely impacted my eldest who had multiple interactions with the students at one particular Uni and she did not like what she saw. Offer Holder days can give you more opportunity to meet the actual and prospective students