Number 18 inside a red crossed-out circle, an adults-only, NSFW, over-18 age restriction warning sign.
768 characters in the Miscellaneous Symbols and Pictographs Unicode block, from number 543.
U+1F51ENO ONE UNDER EIGHTEEN SYMBOLU+1F51FKEYCAP TENU+1F520INPUT SYMBOL FOR LATIN CAPITAL LETTERSU+1F521INPUT SYMBOL FOR LATIN SMALL LETTERSU+1F522INPUT SYMBOL FOR NUMBERSU+1F523INPUT SYMBOL FOR SYMBOLSU+1F524INPUT SYMBOL FOR LATIN LETTERSU+1F525FIREU+1F526ELECTRIC TORCHU+1F527WRENCHU+1F528HAMMERU+1F529NUT AND BOLTU+1F52AHOCHOU+1F52BPISTOLU+1F52CMICROSCOPEU+1F52DTELESCOPEU+1F52ECRYSTAL BALLU+1F52FSIX POINTED STAR WITH MIDDLE DOTU+1F530JAPANESE SYMBOL FOR BEGINNERU+1F531TRIDENT EMBLEMU+1F532BLACK SQUARE BUTTONU+1F533WHITE SQUARE BUTTONU+1F534LARGE RED CIRCLEU+1F535LARGE BLUE CIRCLEU+1F536LARGE ORANGE DIAMONDU+1F537LARGE BLUE DIAMONDU+1F538SMALL ORANGE DIAMONDU+1F539SMALL BLUE DIAMONDU+1F53AUP-POINTING RED TRIANGLEU+1F53BDOWN-POINTING RED TRIANGLEU+1F53CUP-POINTING SMALL RED TRIANGLEU+1F53DDOWN-POINTING SMALL RED TRIANGLEU+1F53ELOWER RIGHT SHADOWED WHITE CIRCLEU+1F53FUPPER RIGHT SHADOWED WHITE CIRCLEU+1F540CIRCLED CROSS POMMEEU+1F541CROSS POMMEE WITH HALF-CIRCLE BELOWU+1F542CROSS POMMEEU+1F543NOTCHED LEFT SEMICIRCLE WITH THREE DOTSU+1F544NOTCHED RIGHT SEMICIRCLE WITH THREE DOTSU+1F545SYMBOL FOR MARKS CHAPTERU+1F546WHITE LATIN CROSSU+1F547HEAVY LATIN CROSSU+1F548CELTIC CROSSU+1F549OM SYMBOLU+1F54ADOVE OF PEACEU+1F54BKAABAU+1F54CMOSQUEU+1F54DSYNAGOGUEU+1F54EMENORAH WITH NINE BRANCHESU+1F54FBOWL OF HYGIEIAU+1F550CLOCK FACE ONE OCLOCKU+1F551CLOCK FACE TWO OCLOCKU+1F552CLOCK FACE THREE OCLOCKU+1F553CLOCK FACE FOUR OCLOCKU+1F554CLOCK FACE FIVE OCLOCKU+1F555CLOCK FACE SIX OCLOCKU+1F556CLOCK FACE SEVEN OCLOCKU+1F557CLOCK FACE EIGHT OCLOCKU+1F558CLOCK FACE NINE OCLOCKU+1F559CLOCK FACE TEN OCLOCKU+1F55ACLOCK FACE ELEVEN OCLOCKU+1F55BCLOCK FACE TWELVE OCLOCKU+1F55CCLOCK FACE ONE-THIRTYU+1F55DCLOCK FACE TWO-THIRTYU+1F55ECLOCK FACE THREE-THIRTYU+1F55FCLOCK FACE FOUR-THIRTYU+1F560CLOCK FACE FIVE-THIRTYU+1F561CLOCK FACE SIX-THIRTYU+1F562CLOCK FACE SEVEN-THIRTYU+1F563CLOCK FACE EIGHT-THIRTYU+1F564CLOCK FACE NINE-THIRTYU+1F565CLOCK FACE TEN-THIRTYU+1F566CLOCK FACE ELEVEN-THIRTYU+1F567CLOCK FACE TWELVE-THIRTYU+1F568RIGHT SPEAKERU+1F569RIGHT SPEAKER WITH ONE SOUND WAVEU+1F56ARIGHT SPEAKER WITH THREE SOUND WAVESU+1F56BBULLHORNU+1F56CBULLHORN WITH SOUND WAVESU+1F56DRINGING BELLU+1F56EBOOKU+1F56FCANDLEU+1F570MANTELPIECE CLOCKU+1F571BLACK SKULL AND CROSSBONESU+1F572NO PIRACYU+1F573HOLEU+1F574MAN IN BUSINESS SUIT LEVITATINGU+1F575SLEUTH OR SPYU+1F576DARK SUNGLASSESU+1F577SPIDERU+1F578SPIDER WEBU+1F579JOYSTICKU+1F57AMAN DANCINGU+1F57BLEFT HAND TELEPHONE RECEIVERU+1F57CTELEPHONE RECEIVER WITH PAGEU+1F57DRIGHT HAND TELEPHONE RECEIVERU+1F57EWHITE TOUCHTONE TELEPHONEU+1F57FBLACK TOUCHTONE TELEPHONEU+1F580TELEPHONE ON TOP OF MODEMU+1F581CLAMSHELL MOBILE PHONEU+1F582BACK OF ENVELOPEU+1F583STAMPED ENVELOPEU+1F584ENVELOPE WITH LIGHTNINGU+1F585FLYING ENVELOPEU+1F586PEN OVER STAMPED ENVELOPEU+1F587LINKED PAPERCLIPSU+1F588BLACK PUSHPINU+1F589LOWER LEFT PENCILU+1F58ALOWER LEFT BALLPOINT PENU+1F58BLOWER LEFT FOUNTAIN PENU+1F58CLOWER LEFT PAINTBRUSHU+1F58DLOWER LEFT CRAYONU+1F58ELEFT WRITING HANDU+1F58FTURNED OK HAND SIGNU+1F590RAISED HAND WITH FINGERS SPLAYEDU+1F591REVERSED RAISED HAND WITH FINGERS SPLAYEDU+1F592REVERSED THUMBS UP SIGNU+1F593REVERSED THUMBS DOWN SIGNU+1F594REVERSED VICTORY HANDU+1F595REVERSED HAND WITH MIDDLE FINGER EXTENDEDU+1F596RAISED HAND WITH PART BETWEEN MIDDLE AND RING FINGERSU+1F597WHITE DOWN POINTING LEFT HAND INDEXU+1F598SIDEWAYS WHITE LEFT POINTING INDEXU+1F599SIDEWAYS WHITE RIGHT POINTING INDEXU+1F59ASIDEWAYS BLACK LEFT POINTING INDEXU+1F59BSIDEWAYS BLACK RIGHT POINTING INDEXU+1F59CBLACK LEFT POINTING BACKHAND INDEXU+1F59DBLACK RIGHT POINTING BACKHAND INDEXU+1F59ESIDEWAYS WHITE UP POINTING INDEXU+1F59FSIDEWAYS WHITE DOWN POINTING INDEXU+1F5A0SIDEWAYS BLACK UP POINTING INDEXU+1F5A1SIDEWAYS BLACK DOWN POINTING INDEXU+1F5A2BLACK UP POINTING BACKHAND INDEXU+1F5A3BLACK DOWN POINTING BACKHAND INDEXU+1F5A4BLACK HEARTU+1F5A5DESKTOP COMPUTERU+1F5A6KEYBOARD AND MOUSEU+1F5A7THREE NETWORKED COMPUTERSU+1F5A8PRINTERU+1F5A9POCKET CALCULATORU+1F5AABLACK HARD SHELL FLOPPY DISKU+1F5ABWHITE HARD SHELL FLOPPY DISKU+1F5ACSOFT SHELL FLOPPY DISKU+1F5ADTAPE CARTRIDGEU+1F5AEWIRED KEYBOARDU+1F5AFONE BUTTON MOUSEU+1F5B0TWO BUTTON MOUSEU+1F5B1THREE BUTTON MOUSEU+1F5B2TRACKBALLU+1F5B3OLD PERSONAL COMPUTERU+1F5B4HARD DISKU+1F5B5SCREENU+1F5B6PRINTER ICONU+1F5B7FAX ICONU+1F5B8OPTICAL DISC ICONU+1F5B9DOCUMENT WITH TEXTU+1F5BADOCUMENT WITH TEXT AND PICTUREU+1F5BBDOCUMENT WITH PICTUREU+1F5BCFRAME WITH PICTUREU+1F5BDFRAME WITH TILESU+1F5BEFRAME WITH AN XU+1F5BFBLACK FOLDERU+1F5C0FOLDERU+1F5C1OPEN FOLDERU+1F5C2CARD INDEX DIVIDERSU+1F5C3CARD FILE BOXU+1F5C4FILE CABINETU+1F5C5EMPTY NOTEU+1F5C6EMPTY NOTE PAGEU+1F5C7EMPTY NOTE PADU+1F5C8NOTEU+1F5C9NOTE PAGEU+1F5CANOTE PADU+1F5CBEMPTY DOCUMENTU+1F5CCEMPTY PAGEU+1F5CDEMPTY PAGESU+1F5CEDOCUMENTU+1F5CFPAGEU+1F5D0PAGESU+1F5D1WASTEBASKETU+1F5D2SPIRAL NOTE PADU+1F5D3SPIRAL CALENDAR PADU+1F5D4DESKTOP WINDOWU+1F5D5MINIMIZEU+1F5D6MAXIMIZEU+1F5D7OVERLAPU+1F5D8CLOCKWISE RIGHT AND LEFT SEMICIRCLE ARROWSU+1F5D9CANCELLATION XU+1F5DAINCREASE FONT SIZE SYMBOLU+1F5DBDECREASE FONT SIZE SYMBOLU+1F5DCCOMPRESSIONU+1F5DDOLD KEYU+1F5DEROLLED-UP NEWSPAPERU+1F5DFPAGE WITH CIRCLED TEXTU+1F5E0STOCK CHARTU+1F5E1DAGGER KNIFEU+1F5E2LIPSU+1F5E3SPEAKING HEAD IN SILHOUETTEU+1F5E4THREE RAYS ABOVEU+1F5E5THREE RAYS BELOW…and 26 more characters.