If all you want is to apply for Mensa, old scores or the Mensa test are easy and inexpensive options.
If you are looking for a better understanding of yourself, your strengths and weaknesses, and whether your underachievement was related to a learning disability or other issue, an assessment can give you much more than a full-scale IQ score. A cognitive assessment can give you a breakdown of your performance on various subtests, and a skilled psychologist can work with you to interpret the results in a meaningful way.
I have had an assessment as an adult, and it was a valuable experience that gave me new insights.
