I signed up for an IM my first season of doing tris. I did a sprint in May, and Olympic in early June, a HIM in July (on my own- not an actual race) and then my IM in late September. I had done 5 marathons prior to that, but did my first century 2 months before the IM.
Maybe it wasn't wise, but I wouldn't have done it any other way. I've known plenty of people who have done an IM and the marathon during the race was their first.
I think it's more mental than anything. If you want something, you can achieve it. The one regret I had was that I wasn't a strong cyclist. I had only started biking 8 months prior to the IM. Next year when I do another IM I will have 5 years of cycling under my belt and will be much stronger.