No it definitely isn't.
> There's naught deadly in voltage, it's the wattage that electrocutes you.
In fact, if you want to be precise, it is the current (Amps) that kills you. The reason why 24V isn't nearly as deadly is because at the resistance your skin normally has the current will be low enough that it does not get dangerous.
Up to 48V is considered a 'safe' voltage, but you can definitely feel it and DC is far more dangerous than AC (because with DC your muscles contract and stay contracted so it can be very difficult to disengage from the source).
If you lost muscle control over 6V then likely there were some other circumstances in play, for instance you could have been poking around in a spot where your skin had been breached or you may have had sweat or some other salty solution on your skin.
Real danger starts around 80V for DC and 150V for AC. Yes, you can get electrocuted at lower voltages but you'd have to really work at it.
Most high voltage circuits that you are likely to come in contact with have an internal resistance that is high enough that they can not source a high enough current to do damage, that's why you perceive high voltages as safe. But if a high voltage circuit is beefy enough that it can supply a couple of hundred milli-Amperes you're as good as dead because at that voltage your skin is no longer a strong enough barrier.
On a side note, I think I read women have lower skin resistance than men on average.. Something we were going to do a little design of expirment for in highschool but never got around to completing it because shocking people and playing with electricity was more fun.
Ohh.. According to this men have lower resistance. http://van.physics.illinois.edu/qa/listing.php?id=6793
People love love love to repeat this tired "it's not the volts that kills, it's the amps" line, while completely ignoring the fact that low voltage makes it really, really, really hard to obtain the necessary amps to be dangerous.
I'm a little sketchy on the details (been a while since I studied this sort of thing) but I can personally attest to losing muscle control in my right arm on a 6V circuit using only a standard (1Ah?) alarm battery.