PowerShell: 119

Gofled Code

for(;!($x-cmatch'.*(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!-/:-@[-`{-~]).*')){$x='';1..15|%{$x+=[char](33..126|random)}}$x

Un-golfed and Commented

# Start for loop definition.
for(
    # Skip variable initialization, start definition of run condition.
    ;
    # Loop runs if $x does not meet complexity requirements.
    # Length requirement is not tested here because it is enforced by the generator later.
    # Much thanks to @VasiliSyrakis for the RegEx help.
    !($x-cmatch'.*(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!-/:-@[-`{-~]).*')
)
{
    # Reset $x in case the script block has already run.
    $x='';
    # Use ForEach-Object, via the % alias, to run a loop 15 times.
    1..15|%{
        # Append a random ASCII character from 33-126 to $x.
        # Note: Use get-random instead of random for faster performance.
        $x+=[char](33..126|random)
    }
}
# Display $x.
$x
# Variable cleanup - not included in golfed code.
rv x