Because of the increased risks of osteoporosis and heart disease, it is strongly recommended that women who have undergone premature menopause should go onto hormone therapy until at least the average age of menopause, i.e. until age 50 or 51. Premature menopause actually reduces the risk of breast cancer. If you have an early menopause you should therefore be strongly reassured that you can safely take hormone therapy until the age of 50 or thereabouts. Hormone therapy from the time of early menopause until age 50 is not associated with an increased overall risk of breast cancer. It is generally safe to take hormone therapy until age 50 or 51 and the described risks of hormone therapy then start from that age.
Side effects are really no different.
Early menopause is often of unknown cause. It may be due to chemotherapy or radiotherapy to the ovaries, may occasionally be due to genetic factors, and may be due to immune mechanisms. Much work remains to be done on the reasons for it. Stress is not generally regarded as a cause for premature menopause. Hormone therapy for people who have had a hysterectomy usually requires oestrogen only, unless the hysterectomy has been done because of endometriosis, in which case added progestins are also recommended.