a^n where n is odd means..
A language which consists of strings such that.. in each such possible string, the number of a's is odd..
OR
L = {a,aaa,aaaaa,aaaaaaa....} /* There are infinite such strings in the given language */
now your arguement has a problem..
What does n is odd mean..
n can be 1,3,5,7,9... or n is of the form 2p+1, where p =0,1,2,...
So the language should satisfy any possible such value..
When you say we can take n=3,99,79,..
It is wrong because then n doesnot refer to the set of ALL Odd numbers.. it is a proper subset of ODD numbers..