We've brought someone in 2.5 years after starting and given them a sizeable chunk of equity. Not sure whether we'd "call" him a co-founder or not....but it doesn't really matter. They do have a large % of ownership, which is really what matters more. We likely will never have the need for seed or VC money, nor pursue that path, which might make a difference with something like this?
Edit: Another consideration is whether you're paying this new person a fair-market salary.