Indo-Canadian here (based out of YYZ). A lot of my friends and families fly to DEL quite frequently (I usually have someone in my circle going to DEL twice a month). I'm flying AC71/AC72 in April myself.

Based on all the feedback I've received and all the conversations we've had, I would not recommend you leaving the airport. Even if you can escape the traffic, security hassles etc there just isn't a lot to do in Delhi for a few hours. Especially considering the time your flight is, you'd only be able to leave for the city late evening (not good for sight seeing anyways).

If you have a particular restaurant in mind, or a mall, then you'll find yourself continuously looking at your watch; not worth it. Even if you're planning to drive by Red Fort, India Gate, Parliament etc, as I mentioned before, its just not worth it at that time of the day. Okay, I take part of it back, India Gate looks awesome at night time, but thats pretty much it.

Save yourself the trouble, stay at the airport. You're flying J, so you won't have to pay for the lounge. Sit back, relax, take a shower, try some Indian food at the lounge, shop at the duty free, feel free to get me a bottle of Old Monk and enjoy the Dreamliner experience. DEL-YYZ flight has been quite good at getting back to Toronto on time (a lot earlier than expected at times actually, as early as 3:50ish).

TL;DR: Stay at the airport. It sounds messed up, but speaking as an Indo-Canadian, a late evening/night drive to the city while running against the clock won't be a good introduction to India for your 18 year old.