Owning a second home is nothing to aspire to IMO. Just keeps other folks from having one home.
Yes. I would generally have less respect for someone for this, except where it's letting on a philanthropic basis at below market rent, balancing the books in an essential but low paying business such as farming, or people no longer able to do standard jobs because of health issues. (And the latter would inevitably either have had good careers cut short and/or well off relatives to help set them up, so more of the luck which Rosy rightly identified).
The sorts of people described in the OP I would actually stereotype as less grown up than those with less money, because unless they are working in a frontline public service they are heavily insulated from the current experiences of people unlike themselves. (Self-made people who knew what it was like on a low income 30 or 40 years ago might now really grok how things have changed, and too often seem to think others should be able to pull themselves up by the bootstraps without understanding what contributes to this not being possible for everyone.) Obviously there are rich people who are very aware and empathic but there are plenty who aren't too.
I see being grown up as primarily related to skills and knowledge; able to deal unfazed and independently with officialdom, tradesmen, emergencies and so on and stay organised and not exceed a budget. Keep on top of cleaning, able to do some DIY whilst knowing your limits, that sort of thing. Not materially extravagant, even if they can afford it, especially not on things where differences are cosmetic rather than substantially functional, because they no longer care what anyone thinks about fashion, brands and so on.
My less realistic ideas of being grown up in my twenties involved, on top of being capable in dealing with various situations, having very high energy levels and needing little sleep: e.g. all in one evening goes to gym or evening class (during course of the week does both), cooks from scratch, cleans a bit or does personal admin so there's more time for fun at the weekend, talks to friends a bit, watches talked-about TV programmes, writes a substantial and intelligent blog post, reads at least 50 pages of a book. Also goes out 2-3 times a week during the week to bar/ cinema/ exhibition with no adverse effect on work performance or state of home. But the great thing about getting older is that feeling so led by the media seems to diminish, most people don't seem to be doing as much anyway and so it's fine if you don't.
The things that still get in the way of fulfilling my own ideas of grown up are all health-related. If one condition got a lot better I would feel I was there according to my own principles. That sense of feeling grown up only started after I'd turned the age my mother was when I was born, which must have something to do with it.