He (almost certainly) castrated himself as a young man, (
Matt. 5:27-31), but I could find no exact age, plus his writings say nothing about the matter. This history is considered
'probably true' and is corroborated by several of Origen's contemporaries.
To be fair, as you at least alluded to, whether or not Origen castrated himself is not a settled matter. I know there is close to a historical consensus that he did, but the arguments that can be given against such having been the case are considerable.
First, the primary evidence of it is a second-hand account from one Demetrius, a semi-hostile colleague, as later reported third-hand by the church historian Eusebius (
Ecclesiastical History, 6.8.1.ff). However, Eusebius’ account is somewhat contradictory. He claims Origen did it partly to be free from slander (as he was a teacher of both young men and women catechumens), and then says he couldn’t hide it, however hard he tried. But if the reason for becoming a Eunuch was to avoid slander, wouldn’t you make sure everybody knew about it?
Eusebius also claims that in addition to avoiding suspicion while teaching girls, Origen castrated himself because he took Matt. 19:12 too literally. But Origen is probably best known for his allegorical, non-literal interpretation of Scripture.
Perhaps most importantly, Origen himself wrote: “Let us suppose there is a difference between church and synagogue. In its proper sense the church has no spot or wrinkle or any such thing, but is holy and blameless. Into it enters no bastard or eunuch or one castrated.” (
On Prayer, XX.1; PG 11:477) Moreover, this was written after Origen had moved to Caesarea, meaning that unlike many of his other writings, it was not subject to having been corrupted by his primary translator into Latin, the notoriously unscrupulous Rufinus.
I of course do not agree with much of Origen’s theology. In terms of universalism, however, it is notable that one of the much revered Cappadocian Fathers, Gregory of Nyssa, taught it as well.