And I think it's even a little more than this. I've certainly seen college grads come up with impressive algorithms and "solve hard problems". For me senior is having seen other people solve hard problems in the past and fail spectacularly when their math meets implementation details. Maybe the approach requires too much precision from data entry clerks., Maybe it's particularly sensitive to GC pauses or doesn't distribute well given potential bandwidth issues between DC's. Maybe its a awesome but future devs will find it so terse as to be un-editable. A lot of being a senior to me is having seen many production systems and learned the weird ass ways they can go wrong.

Still not saying you can't do that at 18, but you probably needed to quit high school and start consulting for teams at 14 to get the experience in by then!

The word itself implies more experience than others. More experience is usually correlated to length of your tenure. But yes I agree, other qualities of senior developers include what you wrote.

Just being a devils advocate here: I was 14 leading the school programming club and I thought I solved really difficult issue setting up gentoo. Senior at 14?

Senior within the context of the club, yes.

Depends on the culture of the country many counties have custom and practice that you have to do X years as a grade before you can move up.