They can stay in fresh water with a high PH (8+) for a while. I'd say by 2 years they need high end brackish. They don't ever need FULL marine, maybe about 1.002 ppm.
I have three Columbian Sharks I keep in an SG of about 1.008. I have been slowly adding more marine salt with each water change to gradually increase the salinity. Within the next four or five years it will probably be about full marine. They are doing well and growing fast. Just know to never increase the SG by more than about .002 per week or it will kill off the beneficial bacteria in the tank and you will restart the cycle. Fortunately these are very hardy fish and my sharks have cycled three tanks as juvies before they found their home in my 55. By next year I will have them in a 210.
