The Attorney General decided not to refer Rolf Harris' sentence to the Court of Appeal because there were several reasons the original sentence would not be considered "unduly lenient".
The entertainer's age was one of the factors that influenced his sentence of five years and nine months for 12 counts of indecent assault.
A spokeswoman said the judge in the case also had to sentence Harris based on the maximum punishment available at the time of the offences.
It was also not possible to "simply add up sentences on individual counts" to reach a longer sentence, she explained.
"The overall sentence had to be just and proportionate to the overall offending," the spokeswoman added.