大龄 literally means 'big age' and refers to someone who is older than what is considered typical for a certain life stage, especially marriage. It's most often used in the compound 大龄青年 ('older unmarried youth') to describe people in their late 20s or 30s who are still single. The term carries a social expectation that people should marry by a certain age, so it can sound slightly judgmental depending on context. Use it as a noun that modifies other nouns (e.g. 大龄问题 'age-related issue').