Keeping in mind I’m not a veterinarian, I’d say yes if this were a breed less massive. By 18 months, on average, the growth plates are typically said to be properly fused, but I’d ask a veterinarian to be sure, as the larger breeds like the Cane Corso take longer to mature physically and it might be better to wait.
Be sure that the vet isn’t giving you a ‘pat’ ‘on average’ answer and actually has some experience with the Molosser breeds and this sort of activity. My guess is depending on how hard-core and intenselly you plan on pursuing this you’ll be told to wait until at least 2 years. If it’s just once a week a few circuits, probably no problem if it’s the low impact stuff. Let me know what you learn.
FWIW, I don’t recommend agility for a Cane Corso. It’s not the activity, it’s the exposure to other dogs and people in a competitive off-leash setting, combined with the guarding genetics that can really catch Cane Corso owners off-guard somewhere between their Cane Corso hitting 18 months and 3 years of age. Particularly the males.
Your obedience would have to be rock solid and many (but not all) agility trainers bizarrely discourage obedience at least in the proximity of and in the agility ring setting, so you will find your Cane Corso being constantly exposed to dogs unable to exert self-control and jonesing for the next treat.
I would recommend pursuing an alternate enrichment outlet more in keeping with your dog’s genetics.
John ‘Ask The Dog Guy’ Wade