French long-stay visas typically need to be validated within three months of entering France (there are exceptions, including working holiday visas). This used to entail a medical checkup and getting a sticker but can now be done online. What happens if you exceed that delay is not completely clear. In theory your presence could be deemed illegal which would in turn create many problems down the line but I have also heard of people who submitted their application a little late and didn't suffer any consequences.
Note that this is three months after entering France, not the Schengen area. A long visa-free visit elsewhere in the Schengen area before could therefore create some difficulties because it would prevent you to use your entry stamp to document your date of entry or could make it appear as if you went past the three-month deadline. A week is not long enough to be a problem.