The youngest cast member in the history of Saturday Night Live is Anthony Michael Hall. Yes, the same Anthony Michael Hall who made his name playing nerds in classic(?) 80s high school movies spent a year in studio 8H.
So, Hall's career peaked at that age of 17. Few remember that 1985-86 season, for it was utterly terrible. (Although it did give us Jon Lovitz, so there was a pear-shaped silver lining.)
During that season, Hall was often paired up with Robert Downey Jr. Yes, Ironman himself, one of today's most respected actors, was on SNL as well, also during that awful season. He was just 20, making him the third youngest cast member ever.
Fourth youngest? Pete Davidson, currently on the show. He is also 20, but had a few months on Downey at the time of their respective debuts. Will he rise to Downey's heights or drop to the depths of Hall? Time will tell.
Oh, right, we skipped second youngest. Debuting at the age of 19... Eddie Murphy.
The world is wide open to possibilities, young Pete.