Citation :I have lived in the same apartment for 10,5 years. I am now leaving. When I moved in I did not register the lease. My copy of the lease does not have a stamp to show it has been registered.
How can I find out if my landlord registered it or the agency?
You want to go to the "Bureau de l'enregistrement" concerned by your appartment. None of my tenant have any stamp from the "enregistrement", but all my "contrats de bail" are registered.

Citation :The landlord had is own expert carry out the etait des lieux, which I signed when I moved in.
If the lease was not registered, is this etait des lieux valid? What should count as age after 10 years? The carpets and walls have never been painted/replaced in that time so I think it's not necessary to clean them so well...
Registered or not, the "état des lieux" is valid.

If there is a expert, he will know what is due to the age of things, and what is your responsibility.

Just an additionnal note: lack of cleaning is never caused by "age", but by a lack of maintenance by the tenant. Always charged to the tenant...