KF, these are excellent questions.
I have a feeling that 18 years old on the first day of the year is the youngest you can have an entry for any Mountain Course race. But I don't actually know!
I reckon that Arthur Wheeler is the oldest rider to have competed in any event on the Mountain Course, at something like 77 years old. It was in the Classic Manx in the late 80s, but once again, I don't actually know.
The only rider that I know of who is over 70 and competing is my good friend Keith Dixon of Accrington, who rode this year, and was born on the 1st April 1943, but I reckon Arthur Browning must be a close 2nd.
Roy Hanks must be the oldest current TT competitor, who is (I think) 66years old.
There are a few 18 year olds to have ridden the TT. Mike Hailwood is one, and Stanley Woods is another. I'm sure there are more.