
The score freezes.
On the other hand, the score only matters to get your ITA. Once you have the ITA, why do you care anymore about your score?
Just get all your papers within the 90 days limit to be on your way to receive your PR.
Edit:
Also, if you have a birthday after receiving your ITA, it doesn't matter, your score won't go down and you still are qualified.
However, if you have a nomination from a province, or a job offer, receive your ITA, but then you no longer have one of those, you have to update your profile, it is your duty to do so. And if it goes down, then I think you'd have to decline the ITA.
You should confirm that, but they are very strict about misrepresentation, so be careful anybody that may have one of these situations.