Two Birds On A Wire by Regina Spektor.

Age 7 and 4
Sam and Tim sat together on a hill. Sam tugged at his older brother's sleeve, wanting to go exploring away from home.

Tim glared at him.
"I want to but... I think we're a bit young," Tim explained. But, he was a liar.

Of course, Sam believed him, telling him he'll always understand. Sam grabbed his hand, holding tight, promising to never let go.

Age 10 and 7
Sam and Tim sat together on a hill. Sam shook his older brother's shoulder.
"Come on!" He yelled playfully.

"I'm tired," Tim sighed, "the sky's overcast. I'm sorry. One more or one less. Nobody's worried."

Sam groaned, sliding down the tree. "Ok, I believe you. Just remember, there's nothing I won't understand," Sam explained, forcing a smile, and holding his brother's hand.

Age 14 and 11
Sam and Tim sat together on a hill. They remained close together. Constantly, they promise to stay together. But, Tim was never going to let go of the past or try new things. He says that he will, but he was just a liar.

Age 15 and 12
Sam and Tim sat together on a hill. Sam pulled on Tim's arm, wanting to explore.

Tim glared at him.
"I want to but... maybe later," he mumbled, turning away. But, he was a liar.

Age 25 and 22
Sam and Tim were together on a hill. Sam walked away, while Tim just sat there. He reached out half-heartedly, but soon dropped his arm. He had missed so many opportunities. Sam wasn't coming back. The hill was left alone to Tim.