So herpes lives in the nerve set that supplies an area from your waist to your mid-thigh. The virus can travel on any branch of that nerve, showing up anywhere in that area – genitals, buttocks, anus, belly, thigh.
Even if you have never had an outbreak on your buttocks before, it still could be that this in not a new infection for you and unless the blood test was done immediately with the outbreak, you can’t know if the antibody was present because of a new infection or an old infection.
If your partner tests negative for HSV 2 antibody, then you will know that this did not come from him and that you have had the infection for a while.
Your age is not a factor in any of this.