As human longevity increases, the proportion of older people worldwide increases as well. Statistics show that, while the population of older adults (ages 65+) was 5% in the 1960s, in 2018 it increased to 9%, and it is projected to rise to 16% by 2050. According to the World Health Organization, the proportion of the population aged 80 and over is growing fast. However, older learners have been neglected as a cohort by researchers. Although there have been more studies on older adults' language acquisition recently, there is still a lack of research about the benefits and challenges of learning a second language in late adulthood. For these reasons, this paper will begin with a summary of background studies on the age factor in second-language learning, and then discuss its benefits and challenges for both older learners and language teachers.