How long does someone have to sue a doctor?

Please permit me to expand a bit on the answer provided by MD. The "discovery rule" in PA specifies that you can sue within two years of when you know or should know by the exercise of reasonable diligence that you have a cause of action caused by the negligence of the doctor involved. This applies if the action is based in medical malpractice.  This discovery rule also applies to a minor.  Therfoere, the minor has two years following the 18th birthday as a general matter and is also entitled in the appropriate situation, to the application of the discovery rule.

The time period in which you have to sue for a claim is known as the "statute of limitations".  These statutes not only vary by state, but they also vary by cause of action.

A personal injury action must be filed within 2 years of the date when the injury occurred; this includes medical malpractice and personal injury claims. Pennsylvania does, however, have a "discovery rule," which, in limited cases, extends the time for filing a personal injury action.  Under this rule, the statute of limitations will begin to run at the time the plaintiff knows, or should have known, that he or she was injured and that the injury was caused by another's conduct.

Note:  Except in cases of wrongful death, an "unemancipated minor" must file suit within two years of his or her 18th birthday.