There's probably two reasons for this. The obvious one is that the person is more likely to have relevant experience in some field were they probably identified an issue they are trying to solve. This is even more true in the B2B world were a early 20s kind of person simply didn't have the exposure to critical pain points.
The second more subtle point is that someone older is likely to be settled (job/family/money) and switching to a startup requires more intrinsic motivation to do it for the "right reasons". If you have two kids and a somewhat set job at a FAANG or big corp or whatever there must be something really bothering you to give that up. So ceteris paribus I'd assume the older person is far less likely to come in there because they want to "work on something cool", "yolo-startup can still rebuild later swing for the fences" or "this is the path to all the moneyz" and more likely to come there because they see the value of solving the problem they have identified and assume all else will sort of follow.
The second more subtle point is that someone older is likely to be settled (job/family/money) and switching to a startup requires more intrinsic motivation to do it for the "right reasons". If you have two kids and a somewhat set job at a FAANG or big corp or whatever there must be something really bothering you to give that up. So ceteris paribus I'd assume the older person is far less likely to come in there because they want to "work on something cool", "yolo-startup can still rebuild later swing for the fences" or "this is the path to all the moneyz" and more likely to come there because they see the value of solving the problem they have identified and assume all else will sort of follow.