In modern times, a woman is considered to be a spinster after childbearing age, around age 40. According to Oxford Dictionaries, a spinster is an older woman beyond the usual age for marriage. The age of a spinster has changed over the years.
The term spinster was once the legally recognized term for an unmarried woman of older years. Historically, a woman was typically married by the age of 20 and may have been considered to be a spinster at the age of 25. The term is still used but usually in a derogatory way to describe a sour woman who is unable to find a husband.