That equation for approximate IQ is incredibly inaccurate for a few simple reasons:
Firstly, it doesn't account for mental decline after about the age of 26. A better equation would be more complicated than I care to post given the meager formatting options available here, but it could be put together to account for this.
Secondly: It assumes linear development of IQ. By this equation, I would have an IQ of 200, as I was thinking like a 4 year old at age 2, a 10 year old at age 5, etc. The problem with that is, I most certainly do not have an IQ of 200. To suggest that would be hyperbole bordering on lunacy. Most people who have such absurd mental age as children also peak in their mental development much earlier, more than likely their development significantly slows around ages 13-15.
TL;DR: IQ equation based on mental age is useless because of nonlinear development and mental decline starting in mid-20s.