I agree with Onyx, at that age it's not something they're likely to 'grow out of'. My in laws found the dedicated digging space worked well for their old dog, he had a patch he'd go hide all his bones and stuff and left the rest alone. My parents' dog didn't get it though, that it only had this one patch to dig in - she figured if she could dig there then she could dig anywhere.

What Dad did was put a trap type thingie (those big ones that snap together) upside down in the hole, and cover it with newspaper. Then he covered the hole again. If you do this, test it first so it will just jump up, not hurt the dog. After finding two 'jumping holes' it scared the dog into not digging again. Don't know if you'd like that option though!