I can sort of see your point but it's a bit of a straw man argument because it can be applied to pretty much any other circumstances.
E.g. if you don't have a job you're seen as a freeloading loser even if there may be very good reasons why you can't work, but by getting a job you're suddenly transformed into a productive member of society.
If you're a single mother you're judged even if the relationship broke down through no fault of your own. But stay with your partner and you don't get the same judgement, even if actually it would better for you and the kids to have split up.
If you don't have kids you're judged even though you either might not want them or be able to have them.
There are millions of other examples for anyone who deviates from the "norm" in any way. Which realistically is most of is in some way! I don't see why your single person living at home is more judged than any other else.
Also it isn't just a binary of living with parents and being single/living with partner. Lots of people are single but live alone. Some people are in relationships but haven't moved in together yet.
ignoring the relationship aspect, firstly for most people living separately from their parents, whether alone, with friends, or with a partner is more independent than still living with parents. So it's not an unfair judgement. Doesn't mean someone still living with their parents is necessarily immature but it is often a less independent option. Also its very rare for someone to still be living with their parents by their 30s, even now (think it's about 3% of women) so people will judge unusual situations, whatever they are.