Less old Shares are good.
But how can the miner reduce that himself?

Should not the miner just work and send all found shares to the pool? ... and the pool "decides" if the found share came too late?
Does the miner not send all shares to the pool, but decides after a self-invented logic whether he sends it or not?

Miner can only change pools and find one that matches his hashrate. I think it is mostly the pools fault.
I went from ethermine and 40 to 60 msec pings and tons of stales to anorak with 120 msec pings and no stales. Zero.
This has nothing to do with changing the pool.
Readme.txt - FEATURES
- Effective Ethereum mining speed is higher by 3-5% because of a completely different miner code - much less invalid and outdated shares, higher GPU load, optimized OpenCL code, optimized assembler kernels.But how is this achieved? By better code or a logic in the code that decides that a found share might be outdated, because it was found, for example, only after 40 seconds.
Don't believe me but please try it yourself. I am not talking about stales reported by the miner but rather by the pool.