I have a 72 bowfront that is about 9 months old now. I have had my mandarin in the tank since October and she is fat as can be. I agree with
@OrionN the tank age is less critical than the fish you have in the tank currently. I started my tank from day one with a mandarin in mind. I started the fuge at day 7 and at day 14 started seeding pods. I also started a culture in a 10 gallon tank around month 2 and dos phytoplankton in my DT and Fuge daily. Anyone who has run a successful culture will know that the pod population reproduces at a very fast rate and as long as you don’t have too many pod eating fish the population will flourish. I’m actually having an issue with too many pods and my mandarin cannot keep up with them. I have a wrasse in QT right now that I will be adding to the DT in a couple of weeks to try and keep my pod population in check. So in my opinion it is doable this early in your tank as long as you have the pod population to support the mandarin