Pre mature education has two different reasons behind it - medical or psychological. If your partner is medically fit then most probably the reason is psychological which can be treated easily with proper counseling and techniques and if required very less medication.
Next Steps
Consult a psychiatrist nearby or online so that thorough evaluation is done and root cause identified. Once that is identified it can be treated accordingly
Health Tips
Ask your partner to start practicing meditation and exercises which will help in reducing stress.