When I got my IQ score in elementary school it was explained to me in quotient terms -- if your IQ is 150 and you are 10 years old you are as smart as a 15-year-old. Mental age stops increasing after the midteens, so this definition of IQ is not universally applicable.
Once I had a discussion with a school counselor where he said "we usually don't let kids of age X do Y", and I replied, "given my IQ, my mental age is actually much higher than X and I should be allowed to do Y". He thought this was arrogant and did not agree.
So if you tell your child she has a high IQ, she may decide that is able to learn things and make decisions usually reserved for much older children. I think this attitude is generally valid, but it could have drawbacks. My 10yo son has not taken an IQ test, but his SAT math+reading composite score overlaps with the scores of students at good 4-year colleges. I think he can study some subjects at the college level if he puts his mind to it.
