That's easy. The only really true answer is your age. Because, smoke and heat can be trapped and made to go around and around in a room. A person's height can, with age, diminish.
A helium balloon can still have helium in it when it begins to descend. But, a person can never become younger. They may look younger with help from plastic surgery or chemicals. But, their true age can never be taken away.
I think the answer is time because when time is passed it never comes back again. Age can also be the correct answer but height is not the correct answer because when a person gets older, their height changes.
The correct answer is age because as soon as you are born your not one yet but the next year you will be one you have aged from just being born to being one years old so however said height is wrong because people shrink when they get older or they just stop growing and become smaller or shorter but all these answers are wright because all of you said age in your answers every person is a smart and inteligent person.
There is a very common riddle on Rain that it comes down but never goes up. But the one that goes up and never comes down can be a person’s age. Another common answer is a person's height. Heat and smoke are also accurate answers for what goes up and never comes down, due to the fact that heat rises.
However, there could be more answers to this same riddle as height and time also lie in the same list. These three things, age, time, and height don't come back once they are gone until and unless we die and come back to starta new life. That's why it is said about the time, "Time is more valuable than money, you can get more money but you cannot get more time".