SignlessAcolyte: When do you want think that Zuko and Azula would retire from being the current Fire Lord and Lady? In their 40s or 50s to give their children a chance to rule in their prime, while they relax and explore the world without any plan in mind? Or do they work well into their twilight years?
"Retirement" for them would be a very slow process. They'd teach Izumi (and the other kids, just in case) to be a good ruler, and let her start making decisions with them once she an adult, until she's essentially ruling alongside them. Only at that point, with her already acting like Fire Lord for enough to prove she's ready to do it by herself, they'd back off...
Well, mostly. Obviously they'll always be more than willing to help her out if she needs and they go back to their old roles as Fire Lord and Fire Lady if Izumi has to leave the nation for a little while for whatever reason, and Azula is going to be acting as 'Izumi's mentor' and asking for updates on everything that is going on until the day she dies because Azula's brain just cannot stop being on Fire Lady mode.
And of course, they'll always help their other kids, and their grandkids, in anything they need.