So, Google says that the regal blue tang can live between 8 to 20 years. That's a big range.
In the movie Finding Dory, we know that it took just a few moments for her friends in the Marine Institute to remember her and her parents, so we can assume it was not that long ago since she was there. Also, her parents don't look to much different, comparing Dory's memories and when she actually find them again.
Since Finding Dory takes place just one year after Finding Nemo, knowing Dory's age in any of the movies will answer my question.