Dora is 26 in winter '04 (144), and evasive about her age in spring '05 (1928), so she's 27 currently. Sven is a year older (337), so 28 currently.
Steve is 24 in spring '04, so he's 25ish currently.
Marten graduates college in spring '03 (162). Faye moved to Northampton in autumn '03 (458), which was "two years, give or take" after her breakdown, which was itself "the summer between sophomore and junior year" (507), so summer '01. Claire is 24 in summer '05 (2281). So all these characters are the same age (24, currently).
Hannelore turns 22 and Tai graduates college in spring '04, so these two are a year younger (23, currently)
Clinton is 21 in summer '05 (2281). Emily is at least a year older (a graduate student, per Claire's comment in 2205), but young enough to have taken a class with him (2853), so she's probably 22.