If they say 'no', leave. Tell someone where you are going and when you should return. Never cave alone. Recommended group size is three people. Three sources of light, including a headlamp attached to a helmet.
Caves are more than just dark, there is no light.
No matter how long you wait, your eyes will not adjust to the dark. Wear comfortable foot gear appropriate for caving. Boots or sturdy shoes with lug type soles work well. Wear gloves and warm clothes. Bring enough food and water for an unexpected stay in the cave.