That seems very unlikely. Agnus doesn't seem that old, for him to have already been deep in the Demon stuff before 3 even happened.
And remember, the plot is basically kicked off because the Order was out hunting for Demons and Devil Arms to power their crap, I don't think they were doing it for like 15-20 years before anyone noticed.
I can't pinpoint age of Agnus, he appears to be middle aged. If we take a wild guess that he is 45, then minus 20 would make him 25. Still enough years for a genius to get some starting experience.
Or are you saying he was sitting while twiddling his thumbs, and then suddenly had a boom in knowledge big enough to be opening hellgates and conducting trials with ascension ceremony on knights years before DMC4?
The plot is kicked off cause the knights butted in on Lady's job, it was probably them becoming bolder in their hunting after draining most of the resources on their own island and area in close proximity (we have zero clue how far Fortuna is from Dante's town, but they don't seem that very close).
Arkham was conducting experiments on demons without his own wife and daughter knowing, despite living in the same house, showing that if you know how to hide it, it won't be that easy to be found out by undesirable party.