You can't really tell the exact age based on size and if cared properly datnoids live for a very long time.
If turning a profit is what drives you in this hobby than good luck to you. The black ray prices seems to drop by the day and I'm pretty sure that neither food or electricity is getting cheaper.
When someone lacks confidence when it comes to keeping fish alive it's a good sign that they have no business owning that fish in the first place. And a fish is not an "investment" unless you are planning to resale it.
you must of been offended with my post but just to tell you its not lack of confidence I'm just saying that anything can happen to it OUTSIDE of your control and what I meant by investment is just the process of purchasing it to clear things up if you want to get all "technical" about it, but if you like dropping 2k per fish then by all means do it I just feel I can do something better with that much money and get something I will long enjoy after the fish is dead due to old age.