In fact, this name is quite uncommon; it's not like it sounds entirely archaic and unfamiliar for a modern Ukrainian ear, but it's not even among top 50 male names (according to 2013 database study by Privatbank). From my experience I would say that most of Semens are 40+ and I don't know anyone bearing this name personally.
Meaning: Karelian variant of Simon.
http://www.nordicnames.de/wiki/Semen
http://verkkopalvelu.vrk.fi/Nimipalvelu/default.asp?L=3
(Information from name #238435 originally submitted by user cutenose)