Number + 岁
龄 (líng) is a bound morpheme meaning "age" or "years of." It does not stand alone but combines with other characters to form age-related words, such as 年龄 (age), 学龄 (school age), and 岁 (as in 40 岁, meaning "40 years old"). In the pattern Number + 岁, 岁 indicates a person's age.
你知道她的年龄吗?
Nǐ zhīdào tā de niánlíng ma?
Do you know her age?
我们家小孩已经到了学龄,该上学了。
Wǒmen jiā xiǎohái yǐjīng dào le xuélíng, gāi shàngxué le.
Our child has already reached school age and should start school.
他今年三十五岁,正是事业的黄金年龄。
Tā jīnnián sānshíwǔ suì, zhèng shì shìyè de huángjīn niánlíng.
He is thirty-five this year, right at the prime age for his career.