Lawnmower is not starting and is now leaking gas
Once you fix the carburetor leak, try to start it. Common problems are:


air filter dirty- remove the air filter element and replace it or clean it thoroughly.
carburetor jets plugged- you said you "cleaned" the carb, but did you completely disassemble and make sure the jets were free of gummy varnish and residue?
old fuel- you state that fuel is "1-6 months old", 6 months is too old. You should drain all the fuel and replace with fresh.


Gasoline in most areas now contains a percentage of ethanol (ethyl alcohol) which is bad for older fuel lines and other components. You do not say how old your mower is, but you should inspect and replace any fuel lines that feel soft or gummy. Also, drain and clean your fuel tank and lines, flush with fresh fuel. Ethanol tends to absorb water from the air which will settle at the bottom of your fuel tank over time. You should try to use only fresh fuel (no older than 3 months) in your mower.