Back in the olden days you had to be 21 to hold an ATC licence.
I don't know what the regs are these days.
Radar coverage at useful levels maxes out at around 160 nm.
It is limited by line of sight and the curvature of the earth.
It's a bit hard to have radar heads floating in the ocean so no coverage out in the indian, pacific, southern oceans or the coral sea.