How old is the senior age

I think it's 65 years.

Costco considers ages 60 and older as Senior Citizen range. Although, I doubt Costco checks IDs - unless you look out of place and don't look like a Senior Citizen. For their Senior Citizen customers, they open up their doors an hour prior to their usual opening time to allow their Senior Citizen customers early access. That way those Senior Citizen customers can avoid the large crowds... which is a pretty good thing.

I think its 60

Costco doesn't have a senior discount according to what I have found.

I'm pretty sure seniors need to be 60 plus.