the parasites load will definitely increase as the fish get older, and this is also based on large wild specimens. smaller captive specimens will either have them and die or they will survive. i highly recommend against de-worming at that small of size and young of age, but that's not to say it can't work...from time to time. as you said, later would generally be better. prazi-pro is what i use on my gars/bowfin if/when they get sick, but i have never used anything as a preventative measure (treating only if there is injury, visible parasites, signs of infection, etc). primitive fishes just don't respond to meds in the same way more derived fishes do, and therefore the risk is usually not worth it unless absolutely necessary IME--
--solomon