The one basic attribute ...
If your CV says you're over 40, then every contract becomes 10x harder to get.
Not because the end-company, who will be using your services necessarily has any problem with "oldies" (they may even have one or two of their own), but because the agencies won't put you forward - unless they have literally no other option.
Phrases like "I don't think you'd fit in", or "The client wants a dynamic team" will be stock replies to your enquires as to why you haven't heard back from the latest application, that was a perfect fit to your CV and what you can actually do.
Whether the client did brief the agencies to "lose" all the applications from
more experienced older applicants , or whether they took it upon themselves to perform this extra service for free is something that will never be made known. But if challenged they'll tell you that contracting is a young-person's game. While this is patently bollocks, the hardest part of getting a contract for anyone with a grey hair is getting past the gatekeeper and actually making it to the client interview.