I actually thought that Castle was born in... 212 SA? By virtue of taking the current year and subtracting it by his biological age. That is until I realized that I hadn't taken cryosleep into account, which means he was born at least in say, 197 SA (taking the 15-year journey to Proxima Centauri into account)? We'll be generous and give him say... 5 years of cyrosleep experience (traveling around the outer planets extensively, but rarely ever making long voyages to Earth or the other inner planets)? That would mean he was born in 192 SA.

Roughly.

Now assuming that the age of service on Titan is eighteen for simplicity, then that would mean Holden was military since 210 SA. I listed him with decades of military experience, and he was only really retired for at most, a couple of years before going back to serve on the Pandora, so, taking that with the year of exit of 249 SA, with up to 2 years taken off to reflect early retirement, then that'd mean Castle has served for 37-39 years in the SDF (though that's closer to 32-34 years, if you don't count cryosleep years; they do count as Active Time Served though, as per the Van Winkle Laws).

Damn. Now I remember why I just decided to slap on "chronologically older" on the age section instead of thinking of it too hard.

So to answer your question... Assuming Castle was active until 247 SA or early 249 SA, then yes. He would've probably been participating in the hunt for the Outer-Orbital Legion (on the assumption that planetary militaries aided in the search), at least within the area around Saturn. Depending on when the hunt ended though, he may have either seen all of it, or only the initial half, before his retirement (though knowing his propensity to not quit in the middle of a job, he may have seen it all).

That being said, his glory years were during the height of the Saturn Insurrections, in the late 220s / early 230s? With the hunt for the Outer-Orbital Legion being so close to the end of his service, I doubt he would've tangled with the Legion personally unless they've skirmished near the area around Saturn. The potential of Fox having seen the Ajax or even having faced it before is certainly there, though.