As such, they are words that require antecedents. There are these guys I met yesterday. These are beautiful flowers here in your garden. The usage comes up in television ads quite often, but it is not always wrong. Pronoun Agreement Singular antecedents require singular pronouns.
Plural antecedents require plural pronouns.
Cars cruised along the highway, all of them doing more than 65 miles per hour. Each student must do their own homework. A person can appoint a durable power of attorney, which is someone they have chosen to act on their behalf should they become legally incompetent. A person may desire help in ending their own life. The older rule was to use masculine singular pronouns to refer to singular nouns and pronouns of unknown gender.