Do you think that sales really a dead-end job after 35-40? Why?

Not at all.

You can use your contacts and experince once you reach this age. its easy.

of course not, Sales never have a definite age

Age for Sales cannot be defined at all.In all aspects of life ,be it professional or personal,sales are involved.

After 35,what we have experienced can surely be applied for sales,training can be given to freshers based on our experiences. 

Sales at 35 gives the entry leve to the top management, this also gives the confidence of expertize and non failures.