As ЯegDwight says, neither is English. Also, it's than 1 and you must use apostrophe to write I'm... to mean I am... However, even though you write it that way...
I'm a 5 year experienced accountant or I'm a 5 years experienced accountant, it's not proper.
Generally, we say...
I'm an accountant with five years of experience or with experience of five years.
But then you want to use the word experienced. So...
I'm an experienced accountant works fine.
That's because you want that word to be used as an adjective whereas our examples use it as a noun.
I'm not utterly sure about it but let others comment on this usage. However, this usage looks a bit odd -
I'm a 5-yr-experienced accountant which makes the entire set of words an adjective as in a 5-yr-old kid.
Thanks
– user576510 Jul 03 '14 at 10:51