One problem with the $0.age < $1.age syntax is that it repeats the "age", which leads to copy-and-paste bugs. It's also kind of a pain for longer property names.
To take a wild guess, I think a series of key paths / key functions would handle 90% of the custom sorts that are out there today, i.e. only 1 in every 10 custom sorts would need to fall back to the general comparator closure form. Having the sort-by-key form, even a single key, is something I always appreciated in Python and Ruby. We might not have the language tools to do it yet today, though, and I wouldn't want to jam something in to work around that.