Miscarriages are more common than most women realize. Years ago it was often mistaken as just a very heavy period. Now that testing is easy, and early, more women realize that what they're seeing is a miscarriage.
Your ages should not be an issue, for either of you.
Sometimes they happen. Why? It could be due to a number of reasons, sometimes genetic, sometimes due to an infection, often times for no known reason.
By doctor, do you mean your ob-gyn? You have had a complete physical and internal exam, correct? If it continues you may want to seek genetic testing for both of you, just to rule out anything.
Don't give up. Don't get frustrated. Hang in there.