Regulators in Massachusetts decided on Thursday to make bookmakers display particular language on their logos inside sporting arenas. It should be made clear in the message that only those who are 21 or older are permitted to wager. The restriction would apply to bookmakers' standalone logos at locations like Fenway Park, where young people are likely to see the promotion. The vote on Thursday puts an end to several months of discussions and public comments about how to restrict sportsbook marketing initiatives that might persuade people who are too young to place bets.