We'll start out by detailing that an R rating — which stands for “Restricted" — is part of the MPAA rating system, which is applied to movies. As mentioned by Michigan State University, "children under 17 require accompanying parent or adult guardian" to view an R-rated movie at a theater. However, it depends on the movie theater (some of those theater employees wouldn't bat an eye if you brought a bloody giraffe into the movie with you).
An R rating is only second to NC-17. According to the rating system, a theater shouldn't allow anyone under the age of 17 to view an NC-17-rated film, no matter who is accompanying them.
Bridgerton is rated TV-MA, which is the most restrictive television and broadcasting rating that exists. Netflix states that Bridgerton is rated TV-MA for featuring "substances, language, sex, nudity, sexual violence, [and] smoking."