Not really possible to tell the age by size, etc.
It will be okay in that size tank for a little while. You might think of doing the rubbermaid setup since funds are tight - they're so cheap and you can get a large one for next to nothing compared to the cost of a larger tank, etc.
Just feed him/her according to the usual guidelines (prey 1-1.5 times the width of the snakes widest part of their body) and she'll grow fine. Ball Pythons live a long time, so enjoy her and don't rush her growth too much. Just like people, putting on the pounds quick can't be good for them and probably shortens their life span.