prime_dec(n) := flatten(create_list(makelist(first(a), second(a)), a, ifactors(n)))$

/* or, slighlty more "functional" */
prime_dec(n) := flatten(map(lambda([a], apply(makelist, a)), ifactors(n)))$

prime_dec(2^4*3^5*5*7^2);
/* [2, 2, 2, 2, 3, 3, 3, 3, 3, 5, 7, 7] */