Millennials are late 20s maybe even early 30s as upper bound. (My generation). We all get grouped and I see milenials between 22-32. That’s my intuitive feel for it anyway. I don’t know the actual society study on it. I know a lot in my circles getting married now too. Albeit it feels way later then our parents (many had kids when 19-21).
Millenials are, depending on definition, everyone up to the 1980 cohort. So millenials include mid-to-late-30s.
Anyone born between 1981 and 1996 (ages 22 to 37 in 2018) is considered a Millennial according to Pew Research.