You're referring to the 22nd episode of Star Trek: Voyager Season 2, "Innocence".
In this episode, Tuvok is stranded on a moon with a group of humanoid aliens who look and behave like children, and so he assumes that's what they are. However, it's later revealed by other members of their species, Drayans, that they age in reverse, and one of these 'children', Tressa, is actually 96 years old, and very near the end of her natural lifespan.
It's also explained that their bodies lose cohesion after a set period of time, causing them to turn into energy. This is supported by the fact that when Tuvok went into the cave to search for the other 'children' who'd gone missing, all he found was their clothing (a fact you mentioned yourself).
JANEWAY: This child wants to live. We're going to do whatever we can to help her.
ALCIA: It is not a choice, this is her time. Tressa has reached the end of her life.
TUVOK: She has barely begun it.
ALCIA: She is ninety six years old. Tressa, these people are outsiders. They don't understand how we live. They see us as strange because their aging process is reversed. Please, you don't belong with them. I'm here to help you now.
TRESSA: No, leave me alone.
ALCIA: It's perfectly natural to be frightened. You're taking a step into the unknown. The attendants would have helped you prepare yourself. You were never meant to face this time alone.
TRESSA: We weren't alone. Tuvok was here. He stayed with us and made us feel safe. He told us there was no Morrok and that we shouldn't be afraid. He took care of us, even when we didn't behave as well as Vulcan children.
ALCIA: He was right about the Morrok. It only exists in stories. At this age they become easily confused. Their memories are clouded. Near the end of life we reach a state of complete innocence. We free ourselves from all responsibilities to this life and we leave it peacefully.
TUVOK: Tressa doesn't seem to be ready. Perhaps she needs more time.
ALCIA: That's not for you or me to decide. This is a normal biological process which begins the day we are created. The energy contained within our bodies remains cohesive for a limited number of years and then it is released. Nothing can change that.
TRESSA: Nothing?
ALCIA: No. It's the natural course of life.
JANEWAY: Why was it so important that Tressa stay here?
ALCIA: We believe this is where the very first spark of life was created. We are all compelled by a powerful instinct to return here at the end to complete the cycle and rejoin the infinite energy. Tressa, do you feel it calling to you?
Star Trek: Voyager - S02E22 - "Innocence"
After learning of all this, Tuvok accepts that Tressa has reached the end of her life, and accompanies her into the cave so he can be there to provide her comfort and reassurance in the moments before she dies and turns into energy herself.
Tressa's death of natural causes was confirmed in the Star Trek Encyclopedia.
Tressa. (Tiffany Trauberman). Drayan individual who, despite her advanced age of 96, looked and acted like what many humanoid species would consider to be a young child. In 2372, Tressa and several other elder Drayans traveled to one of the Drayan moons in order to carry out their final ritual. Their shuttle crashed on the moon, and all of their Attendants died, leaving the elders alone. Lieutenant Tuvok, whose shuttle had also crashed there, comforted and took care of the easily confused Drayans. With Tuvok's help, Tressa faced her final ritual without fear, and she died of natural causes shortly thereafter. ("Innocence" [VGR]).
Star Trek Encyclopedia
