Yep, this is a fish that, if it attains normal growth rates ala what the species does in nature, should be about 18" long at about 5 yrs old, whereas the largest recorded individuals (of both N. vlamingii and N. lopezi) are about 24" in length (and were probably a few decades old). Having said this, surgeons/tangs along with some other species usually don't reach normal adult sizes in captivity, except in very, very large aquaria (i.e., thousands of gallons, usually in public aquaria). It's really not clear why this is the case, especially given that the animals can appear healthy in every other conceivable way. Nonetheless, that's a fish that will probably reach and perhaps exceed 12" in captivity, as above.
cj