I think the youngest who ever joined were Thanatos and Rough Rider, who were something like 14. (Thanatos predates me, Rough Rider postdates me.) Terra Firma was also pretty young. He was 16 when I joined, IIRC, and at the time was the youngest member. (I was 23 when I joined in January of 2000.) Terra was about a year before me. AcdQueen was also 16 when she joined, IIRC.
But at 18, you probably are the youngest currently.