Possible Clues:

• Unsuitable for tots
• Containing some adult material, per the MPAA
• Definitely not suitable for youngsters
• Not to be seen or heard by children
• Unfit for youngsters
• Ill-suited for kids
• Like 'Wedding Crashers' or 'Bridesmaids'
• Too violent, perhaps
• Risqué, say
• Requiring an ID, perhaps