The possible legal ramification for you is a costly defamaton action and/or (depending on how long ago these illegal activities occurred) an invasion of privacy action being brought by the subject of your biography.
The possible ramification for the subject of the biography is reputational injury.
Everything you write that is reputationally injurious (and saying someone was involved in drug use, prostitution, and petty crimes is all damaging to one's reputation) must be verified and documented carefully. If no arrests were ever made, no charges ever brought, no convictions of guilt ever arising from any charges, you could find yourself facing a lawsuit should you mention any of these crimes in connection with your subject.
Accuracy is vital in a biography.
Then, even when what is written is true, and verified, and backed up with appropriate documentation (ie. arrest records), it does not prevent a defamation lawsuit from being filed against you anyway - truth merely provides you with a complete defense.
Truth, however, does not provide a defense against an invasion of privacy action and, if the crimes are old enough, reviving them can be damaging to a "repaired" reputation, and lead to a costly privacy suit being filed against you.
Bottom line: If your subject has a questionnable past, run all of what you write about it by a defamation lawyer. Then make sure you have adequate insurance coverage for your biography, prior to publishing.