The change in blood pressure as you age is caused by progressive stiffening of the aorta and its main branches. In young people, the aorta is very elastic and expands easily when blood enters it, so systolic pressure is low. The blood that stays in the aorta during the heartbeat moves out of it between heartbeats as the aorta returns to its original size before the next heartbeat. That blood flow contributes to the diastolic blood pressure level.
As the aorta stiffens with age, it expands less when blood from the heart enters it, causing higher systolic blood pressure. Also because of the stiffening, more of the blood in the aorta and its main branches is forced into other smaller blood vessels when the heart beats, thus, less is present between heartbeats to support the diastolic blood pressure, which becomes lower.
Treatment of high blood pressure in older people generally focuses on lowering the systolic blood pressure. Usually, lower systolic blood pressure is also associated with lower pulse pressure. So, even though your diastolic blood pressure is low, your doctor may suggest treatment toward bringing down your systolic blood pressure if they feel it is necessary to do so.