This answer will change based on whether the teens will be there late/have a hotel room or whether they will just be there during the day.
If the teens are going to be there during the day and picked up in the evening, I'd give a cautious yes to being unsupervised, but having at the very least an older teen in the group would be best. If we're talking 12 and 13 or so, supervision would be best. If we're talking 15 or 16 or so, they can probably handle themselves. Set ground rules, such as having to stay in a group, not going outside of designated con areas (walking to the Fairmont and back or going to a gathering in the park should be safe, but wandering around DTSJ might not be) or otherwise designated areas, and/or having to inform someone whenever leaving the con premises with where they are going and checking in when they get there and come back.
If the teens are going to have a hotel room, have an adult there. It's a 24-hour con. Lots of people have parties and alcohol. Walking to restaurants around the area is generally fine during the day (and con goers are usually everywhere during con and pretty good and looking out for people), but can get sketchy at night. Even if you trust the teens to not get into any mischief (which does happen since cons tend to lower inhibitions for everyone but especially teens), there's no guarantee that someone else isn't going to get them into mischief.
Generally, the con itself (as in, inside the convention center) is pretty safe, especially during the daytime. I can't imagine them getting into too much trouble there, especially since teens tend to stick together at cons, and tend to group up by fandom. ("The con itself" does cover a large area, though, as pointed out earlier.) Unless they somehow run in with the wrong crowd or have an unfortunate experience with someone creepy, they'll likely be fine as long as they are generally well-behaved. Really, the worst that generally happens with young teens at cons is that they get a bit rowdy and overexcited and may look back in 10 years and cringe a little. It's not safe enough to leave someone as young as 12 or 13 there without at least an older teen with them, though, I'd think, especially since they may not be mature enough or have enough life experience to make good decisions, and may be overly trusting or naive.
When I was much younger and attending this con, that was many years ago when the scene was different and the con was much smaller. Generally, the rule that my parents had was that my sister (older by ~2 years) and I had to stick together, and we couldn't leave the con itself without a parent with us. I'd say that's a pretty decent rule these days, too, now that there's more people.
Also, while older people at cons will often look out for those who are younger when needed, we're not babysitters and don't always want the responsibility of keeping someone else's kids safe. I had an experience once where I was running an off-site event at night (we got back to the con around 10PM and walked several blocks to a dinner location) and found out /once we got there/ that some of the people in the group were as young as 12 and had no adult supervision. Even the oldest person in that group of people was around 14. I was pretty sketched out that there were such young people at an off-site nighttime event run by a grown-ass adult stranger with no supervision whatsoever, and was super thankful that I was the one running the event and not a predator of some type, and that the kids were relatively well-behaved. (I made the events 21+ after that!)
So I'd say that if the kids are old enough to take care of themselves, it would be okay if they were there during the day, but proceed with caution. If they're going to be there at night or overnight, adult supervision is a must.
And really, if the teens are old enough, supervision doesn't have to be that a parent is there by their side the whole time as long as you make sure they stay in a group and check in occasionally. You can always hit up the hotel bar, the pool, the Tech museum down the street, catch a movie, find fans closer to your age, etc., and still be nearby enough to check on them or go with them places offsite, so it doesn't feel like "ugh, mom/dad has to hover around me all day" for them, and it doesn't feel like "oh my god I'm so old why am I here" for you.
So in a nutshell: during the day, onsite only? Maybe, if the teens are especially mature, but I'd lean towards no. Much better if there's at least an older teen in the group.
During the day, offsite? A little iffy, so be sure to have at least one older person in the group and have them check in with someone else when leaving the con center itself.
At night? Hard no. Have an adult there.
Of course, different people will have different opinions on this, and it depends a lot on the kids themselves. About the youngest that I'd be comfortable with letting go free at the con without supervision of someone older is 16, but I'd still make sure they had someone older around at night. 13 is too young, especially if there is only one teen or if there are multiple teens who tend to have the effect of creating peer pressure and lowering inhibitions or the ability to say no, or that ramp up and create so much excitement with each other that they may not notice danger.