“Does a stellium of retrograde planets (Saturn, Uranus, Neptune) in the 7th house in a natal chart indicate late marriage or no marriage for the chart holder?”
I’ll give you the short answer first: late marriage – perhaps. No marriage – definitely not.
Having Saturn, Uranus and Neptune in your 7th House simply indicates that you’ll need to address your inhibitions (Saturn), commitment issues (Uranus) and delusions (Neptune), before you can partner successfully and with none of those limitations getting in the way of a happy marriage (you didn’t attach your complete birth data, but I’m assuming this question is relevant to you personally). Whether it happens soon or late in life, is entirely up to you. It’s possible that you might have to take time off relationships from time to time in order to figure yourself out on your own, but none of this means you’re doomed to never marry.
As an example, I have Saturn, Uranus and Neptune conjunct in my 6th House. I’ve always had mixed feelings (and a fair share of inner struggle) about the thought of sticking to a soul-sucking, 9 to 5 day job, but this doesn’t mean I never worked a single day in my life – I’m actually pretty industrious under the right circumstances. I just had to find an ideal formula that suits me well without making me feel like I’m slaving my life away while make someone else rich.
Also, don’t forget the chart is to be read as a whole. You didn’t attach your complete birth data, but examining your general attitudes towards partnership (i.e. Venus) and life as a whole, and adapting them if necessary, is inordinately helpful.
Much Love, and good luck!