Of course scientists can be productive above 40 years of age. (that would be preposterous) But in many countries there is already national and international institutions in place to host and nurture senior scientist careers.
Recognizing this need, WAYS has been created originally by UNESCO to help younger scientist careers. There is no magic number and 40 is as arbitrary as 35 or 45 would be.
However, WAYS tends not to stick that much to the 40yo issue and the website is open to anyone with a sound interest for science regardless of its age or its activity.
Indeed WAYS 2.0 would certainly benefit from the contribution of more experienced scientists.

For WAYS activities however especially for meetings and governance, according to its status, WAYS delegates young scientists only.

hope this helps.
gaell