you could do somthing like this and have them enter their year of their birth ex: !1999
the only problem is anyone can lie about their age...

on !@*:join:#:{ 
  if (!$readini(agecheck.ini,AgeCheck,$nick)) {
    .notice $nick Please type in ur Year of Birth ex: !1999 You have 15 seconds to do so before you are banned from this room 
    .notice $nick You will only be asked to do this once if you pass
    .timer.age.check 1 15 ban -k # $nick 2 Failed To Give Year Of Birth
  }
}
on $@*:text:/^!(\d{4})$/S:#: {
  var %age = $calc($date(yyyy) - $regml(1))
  if (%age isnum 18-60) { 
    msg # Oh Ur %age Years Old.. Well Then You Can Stay. 
    .timer.age.check off 
    writeini agecheck.ini AgeCheck $nick $address($nick,2)
  }
  else { ban -k # $nick 2 $iif(%age > 60,TOO OLD,TOO YOUNG) | .timer.age.check off }
}