If the family is happy then why should it matter to anyone else? People have their own preferences about 'red flags' in relationships but I would see MUCH larger ones than this. To best honest, I would be more concerned about large levels of debt or no savings in a potential partner than a person who enjoys living with family and gets along well with parents and siblings.
I think that loneliness is a common reason. Living alone can be very isolating and if you get along well with parents and siblings then that can be much more social and pleasant. Perhaps house shares work for some but I've had lots of friends really struggle in house share situations and still find it enormously expensive. If you don't go to Unit then it can be harder to find house mates. Another important reason is being single. It takes much longer to save up for a house deposit if you're single whereas if you meet a partner it may only take a few short years.
So many people struggle on the renting ladder for years because they moved out quite young and never had sufficient savings behind them. If someone gets along very well with their family, has independence by working, paying a fair share and doing a reasonable share of the housework then I think living at home at any age can work perfectly. It very much depends on the set-up and the personalities of everyone involved. I could not live with my father under any circumstances but I get alone very well with my mother so it really depends.
If a future partner got along very well with his family and had huge savings then I wouldn't be turned off that's for sure 