Hey- is your screen name a reference to Veronica Mars? I adored that show.
I've been in your exact situation... I've taught a number of undergrad classes as an AI, starting when I was not much older than the students (though I'm a fair bit older now... sometimes 10 years older than them. Sigh). Anyway, the worst was when I taught the 400 person lecture class. It was at 8 in the morning, which is a bad time for my IBS. In general, even though I felt queasy and not so great some days I just sucked it up and did it. The first few classes were the worst, and then you get used to it and it gets easier and easier. But the first time I stood up in front of 400 people my hands were shaking too badly to clip the microphone to my shirt.
So all I can say is, it's normal to be nervous, it's probably even good (I'm sure you know that energy feeds public speaking). You'll get used to it and if you've coped before (and even thrived, it sounds like) I'm sure you'll do it again.
As for the age issue, it can be a bit of an issue establishing authority with students that age. It helps that you're male, and just remember that you are there because you have more knowledge than them, and you know better how to guide them in their learning. You are the boss.