AS it's described, he was old enough to start a family with Rampage, and he was young enough to think it was a good idea. An unknown amount of time passes between then and the present, however judging by the state of the Crusaders and how only Charity seems to actually remember Rampage, I would guess that it's been ten to fifteen years since the... incident.

And given that Priest was old enough at the time to get the Crusaders started up, I would put him around twenty to twenty-five at the time, accounting for some time spent wandering since his exile before settling down in Chapel.

So he'd be around 30-40, perhaps a little older.