Online IQ tests produce artificially inflated results. They are meaningless.
In order to have any validity, an IQ test must be administered in accordance with certain clinical standards.
Then there are plenty of arguments about the reliability and applicability of these professionally accepted IQ tests.
For instance, studies indicate that IQ is strongly correlated with childhood environment. Socioeconomic status, quality of education, and various details of home life have been shown to influence IQ.
Studies also indicate that stress tends to lower performance on IQ tests. Naturally, the degree of this effect varies between individuals and circumstances.
Contrary to popular belief, IQ can (and tends to) change throughout a person's life time. I read that one study indicated that as a person grows up, their IQ grows closer to that of their parents at the same age. IQ can also change throughout life depending on various factors that boil down to how you use your brain.
On top of all of this, there are several professionally accepted IQ tests. In the general population, these tests yield similar results for each individual. But in specific populations, such as the autistic population, performance tends to be far higher on IQ tests that are not verbally-based (such as Raven's Progressive Matrices).
To sum it all up: IQ is complex and somewhat controversial. Most people probably have more immediate and important concerns than the specifics of their IQ scores.