My little person Tessa and her husband Ben are 55. And they are started to walk very slowly. Their health bar seems to be good, when they die, would the health go down to nothing?

No, even peeps with a full health bar will die suddenly and with no warning. When their time is up, it's up.

Aww my poor poor Tessa and Ben....with vitiams help?

The slow walking speed is normal with aging. There is a notable change in their walking speed at the age of 55. It's nice if you still have a kid or teen around to help with picking up trash, socks and weeds or even cooking and watering the flowers. Fruits, the bath upgrade, air purifier and exercise are great for health and some say walking on the treadmill will increase their walking speed, but there is still a decrease at 55.