Depends upon what you are seeking to gain from contacting the person. If it was an old friend that you really liked, but because of the years and people moving to different towns, or getting married and changing their names so you simply lost track of them, but would like to re-kindle your friendship, then by all means go ahead.
On the other hand, if it was a person that you had some kind of weird falling out with, even if it was a long time ago, I for one, would rather keep my past in the past.
If it was an old flame, be very careful about what you really want. If you know the person is in a relationship, be very tactful and try not to stir up trouble for him and his current spouse. If you want to get together with him, invite his wife too.
If it was someone you only barely knew from high school, be aware that the person may not even remember you.
If it was someone that you had a crush on, but were never in a relationship with, again, know exactly what it is that you are looking to gain. You or the other person could end up very disappointed and regretful if you mention the “crush” or the person could tell you that they had a crush on you way back then too, and might be interested in persuing you now. Just know what you want, ahead of time, be polite and be prepared to find out news that might shock you.