Joseph, I don’t know which MO and chassidish people you hang around with, but the ones I interact with have slightly different ranges.
I was once by a chassidish family’s house – quite ‘with it’ people, not hardcore chassidish, and the mother was sighing sadly about her single nephew. “It’s heartbreaking!” were her exact words to describe his single status. I was thinking about her nephew, and pictured him as a 28 year old boy whose classmates were all married. Then she said, “How can he be twenty and still single? He has a heart of gold!” I was quite flabbergasted – talk about culture shock! When I asked, they told me that the boys in his class started getting engaged at 18, and now he was one of the last ones left.
Based on other experiences, I do think that many chassidish people would start considering young adults “older” at 21. But it’s different in every circle, and if a girl is 20 but almost everyone in her grade is married, and some had already gotten engaged in 12th grade, then she would be considered “old” as well.
As far as MO, most MO people I know are just starting to get married at 26, which means that it is hardly considered ‘old’.
In case my quotation marks haven’t implied as much, I find this whole concept of 20 somethings being considered old ridiculous, but whatever.