Have a look at
this thread. In particular, following the link in the post by Irv Lee.
You are (understandably) confused. To fly legally you need three things to be valid, and they all have differing expiry times.
1. Your PPL. A JAA PPL is valid for 5 years
2. Your medical - validity vaires with age and Class
3. Your Class Rating - valid for two years
I suspect it is your Class Rating that you are talking about revalidating, not your PPL.
Mike