The only problem at your age with your prostate would be prostatitis, an infection, but that would be very rare at your age. If you were older and had a prostate exam then a doctor would wear a glove and get some gel and slide his finger inside your anus (bottom) and feel if your prostate was enlarged.
If they have said there is something wrong with your butt are you constipated? Do you strain to go to the toilet and does it itch afterwards?
They may look at your butt for that, but very very unlikely prostate at your age.