There's usually little advantage to building a bike from scratch, as bike parts are much more expensive bought separately than as a whole package. If you want a mini dirt bike, go buy one.
But if you insist, you need all the parts that you'd otherwise find on a bike. An engine, some sort of gearbox and a clutch. sprockets, chain. wheels, rotors, brakes. fuel tank, a fork, a headset
THen you need a stack of tubing, tools to miter them, and the tools and skills to make a frame to stick the engine in.
no
you can ride a dirt bike at any age
no
No
No.
yea
In Liverpool
No. Dirt bikes are usually not road registered (although it can be done) so there is no need for any paperwork to sell a dirt bike.
i wanna a 50cc dirt bike
probably
No
You do not normally have to register a dirt bike! But you can call the local DMV or Tag office and be sure. Good Luck