There are a couple of factors that play a role into this. One is that most Ironman competitors rece their best year in their 30's and the other is your speed as one .
If you are in your 20's then I would recommend you develop your Olympic and half ironman distance first and then move up to the full Ironman after a couple of seasons. But if you are in your 30's then I would not wait so long do do an Ironman. Yet still try to peak for a half ironman say in the spring before you do perhaps a end of year Ironman race.