Duh old man said dirty stuff! Har durrrrr!
That must be what was going through the film makers mind when they made this garbage. The entire run time is built upon the fact that usually older people don't speak like that. Once the novelty wear off (about 10mins) the other 90 or so minutes are just a waste of time.
Seriously every joke was just DeNiro saying something offensive, but it's all so expected that it has no effect. It's like when a kid learns a new dirty word and runs around yelling it. You're not offended, it just becomes tiresome after a while...