Assuming you're talking about a specific old and a specific new key then yes, it is equally hard to find the private key (which is incredibly, astronomically, hard).

That said, if you are trying to find a collision by brute force generating bitcoin addresses, then you're more likely to find a collision with an older address because it's likely there are considerably more "older" keys than "newer" keys. It depends on where you draw that line.

Care to explain further?