A person's teens are the period in which they are aged between 13 and 19

Both my daughters are in their teens. 我的两个女儿都十几岁。

He's in his early/mid/late teens. 他十三四／十五六／十八九岁。

the range of temperature between 13° and 19°

The temperature is expected to be in the teens tomorrow. 预期明天的温度介于13至19摄氏度之间。