I tried to find this subject using search but have failed to find it, so if this is a repeat I apologize.

I thought I read somewhere on this forum that the "Celtic levies" were going to start of strong then gradually weaken as the decades go by, is this so?
If this is so what sources is this based on? Caesar, others? Thanks for any information on this, and again if this is a repeat I'm sorry for bringing it up again.